Fix dropped optional arguments in playground
The playground's Variables panel exposes every argument of an operation, but the generated query only declares the required ones. Setting an optional argument used to send a variable the document never references, which servers ignore — the request looked successful but ran without it.
The query's variable declarations now follow the panel: setting an argument declares it, unsetting removes it. Sending an operation declares whatever the panel has set, so a query edited by hand can no longer drop a value. Declarations and values written by hand are never rewritten.
Also fixes the playground's form editing the example rendered on the page, which made Reset restore edited values instead of the original ones.