GDExtension: Ensure that we have automated build CI for Windows and MacOS

This was originally reported by @NicholasBolen in [#351](https://github.com/GDQuest/GDScript-formatter/issues/351#issuecomment-5657302333):

> Unrelated, but Is the standalone version only for linux? I get various errors after installing.
>
> ```text
> ERROR: core/extension/gdextension_library_loader.cpp:369 - No GDExtension library found for current OS and architecture (windows.x86_64) in configuration file: res://addons/addons/GDQuest_GDScript_formatter_standalone/bin/gdscript_formatter.gdextension
> ERROR: GDExtension dynamic library not found: 'res://addons/addons/GDQuest_GDScript_formatter_standalone/bin/gdscript_formatter.gdextension'.
> ERROR: res://addons/addons/GDQuest_GDScript_formatter_standalone/plugin.gd:481 - Parse Error: Identifier "GDScriptFormatter" not declared in the current scope.
> ERROR: res://addons/addons/GDQuest_GDScript_formatter_standalone/plugin.gd:496 - Parse Error: Identifier "GDScriptFormatter" not declared in the current scope.
> ```

@khellste, could you take a look at this?

I'm opening this as a separate issue since it appears to be unrelated to the packaging issue in #351.

まず core/extension/gdextension_library_loader.cpp の GDExtension 読み込みエラーと、plugin.gd のスタンドアロンプラグインのエントリーポイントを確認し、その後、既存のビルド CI 設定を調査します。自動化された Windows および macOS のビルドによって、スタンドアロンプラグインがそれらのプラットフォーム上で読み込める GDExtension ライブラリが生成されれば完了です。

索引モデルが issue の本文から書いたものです。