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.

Start by reviewing the GDExtension loading errors in core/extension/gdextension_library_loader.cpp and the standalone plugin entry point in plugin.gd, then inspect the existing build CI configuration. Done means automated Windows and macOS builds produce GDExtension libraries that the standalone plugin can load on those platforms.

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