[FEATURE] Extend subscription syntax (#790)

Extends the subscription.yaml syntax at the cost of introducing a breaking change.

New Syntax

Mix-n-match presets and indent variables

# Can mix/match presets and indent override variables.
# Uses presets TV Show, Only Recent and assigns Kids, TV-Y to subscription_indent_1 and _2
TV Show | = Kids | = TV-Y | Only Recent:
  "Jake Trains": "https://..."

Subscriptions with list-value support

TV Show | = Kids | = TV-Y | Only Recent:
  "Jake Trains":
    - "https://url.1..."  # Assigns to subscription_value and subscription_value_1
    - "https://url.2..."  # Assigns to subscription_value_2

Subscriptions with override-keys support

TV Show | = Kids | = TV-Y | Only Recent:
  "~Jake Trains":  # the ~ means "all keys underneath get assigned as override variables"
    url: "https://url.1..."  # Assigns to url
    url2: "https://url.2..."  # Assigns to url2

Breaking Changes

In the TV show subscriptions example (https://github.com/jmbannon/ytdl-sub/blob/master/examples/tv_show_subscriptions.yaml), it had

TV Show Full Archive:
  # Sets "Kids" for genre, "TV-Y" for content rating
  = Kids | TV-Y:
    "Jake Trains": "https://www.youtube.com/@JakeTrains"
    "Kids Toys Play": "https://www.youtube.com/@KidsToysPlayChannel"

This must be changed to

TV Show Full Archive:
  # Sets "Kids" for genre, "TV-Y" for content rating
  = Kids | = TV-Y:  # Each indent variable assignment must have an = before it
    "Jake Trains": "https://www.youtube.com/@JakeTrains"
    "Kids Toys Play": "https://www.youtube.com/@KidsToysPlayChannel"