New memory management system

A new document MemoryManagement.md document the new system, but for
existing users of SwiftGodot, there are a number of important
considerations:

• As we have explored in the past, SwiftGodot objects that point to
  Godot objects could be destroyed by Godot behind our back. For
  example, if you create a Node, give it to Godot, and Godot destroys
  it, but you still had a pointer left to it.

  There was no way of determining that this had happened, so you could
  get a crash. Now, when this scenario ocurs, we clear the handle
  in the Godot object, which will do two things: you can now call
  isValid on these objects to determine if the object is still
  valid, and if you forget, a new exception will be raised for you to
  be able to pinpoint those cases.

• In the past, it was possible for tthe same Godot object would be
  surfaced to Swift with different wrapper objects. This worked fine
  because we kept no state on them.

• In the past, SwiftGodot released all objects when they went out of
  scope, following the idiom of Swift and automatic reference
  counting.

  This was not quite correct. With this release, only RefCounted
  objects (Resources-subclasses are one of the main users) get the
  automatic reference counting behavior. Subclasses of Node need to
  be manually released with Node.queueFree, and other non-Node,
  non-RefCounted objects must be manually released with Object.free.

The above was implemented by Gabor Koncz, Miguel de Icaza.

New Variant Binding

In the past, our Variant type would contain the special .nil type,
so when you got a Variant, to test for this, you would need to check
this value. In some APIs that meant first checking Variant was
non-nil, then checking for the .nil state in the Variant. It felt
bad.

This new version of the binding maps a Godot .nil value in a Variant
to Swift's nil, ensuring that if you ever have a Variant, it will
never contain the nil value. This does impact many existing
signatures that took Variant or [Variant], as they become
Variant? and [Variant?].

I have deployed this on a large app, and the results are very pleasant.

This was a contribution that was carefully developed over many weeks
by Elijah Semyonov. Thank you Elijah!

Signals

New @node Macro

Sam Deane contributed a new @Node macro that replaces the old
@SceneTree macro and should be used instead of the flaky BindNode
property wrapper.

Requiring the variable type to be optional or implicitly unwrapped
also doesn't quite make sense - we may as well support it being
non-optional. For non optional variables we'll get a runtime error if
the node is missing, which is also true for implicitly unwrapped vars.

It works like this:

@Node("nodePath") var myRequiredNode: SomeNodeType
@Node("nodePath") var myOptionalNode: SomeNodeType?

The scene tree macro implementation has been tweaked to support
non-optional type definitions, rather than complaining about them. It
does still support implicitly unwrapped types, for backwards
compatibility with @SceneTree, which remains unchanged.

Generic Signal

A new approach to signals that unifies the signals generated by
SwiftGodot. Fixes a long-standing bug #587, and long-standing #42.
Lovely contribution by Sam Deane.

New @signal Macro

Adds a new @signal macro, giving a cleaner and more consistent way to
add user-defined signals from Swift.

@Godot
class MyNode: Node {
  @Signal var mySignal: SimpleSignal
  @Signal var signalWithArgs: SignalWithArguments<Int, String>
}

These signals can be used in the same way as built-in signals:

node.mySignal.connect { /* do something */ }

node.signalWithArgs.connect { intArg, stringArg in
  // do something
}

node.mySignal.emit()
node.signalWithArgs.emit(1, "foo")

The existing #signal macro defines the signal as a static property on the class that contains it.

This is different from the generated implementation of built-in
signals, and means that we have to use a different syntax to work with
them -- which was non-optimal.

Changes

• Now requires Swift 6.0 (Sam Deane)

• Windows build improvements by Sam Deane

• Surfaced support for editorAddPlugin and editorRemovePlugin to
  allow developers to create Godot editor plugins.

• Introduces a new EntryPointGenerator
  plugin for
  all @godot classes in the project (Elijah)

• Various documentation, test suite improvements, warning fixes and
  generator, refactoring changes and internal maintenance and quality
  of life improvements (Chris Backas, Sam Deane, Elijah Semyonov, Rob
  Mayoff, Gabor Koncz, Gianluc Lui, Miguel de Icaza).

• Plenty of new convenience initializers for the Packed*Array types by
  Gianluc Lui.

• New Variant.getNamed(key:) API

• Small cleanups to our sample code (Sam Deane)

• Enumerations no longer generate debugDescription as it was not
  necessary - reducing the API surface (Sam Deane)

• Android support by Marc Prud'hommeaux. Nice tutorial on how to use
  SwiftGodot on the Meta Quest by Tomasz Wyrowiński is here:
  https://github.com/tomwyr/godot-swift-meta-quest-minimal/

• @callable methods now can take optional values #669

• The @export attribute can now specify PropertyUsage flags, if none
  is provided, it will continue to default to .default.

• New support for overwriting the Godot _validate_property method,
  in SwiftGodot, the method to overwrite is _validateProperty and it
  can be used to change the property usage flags of a dynamic
  property.

Fixes

• #581
• #579
• #613
• #624