GDScript enum fixes & refactor

[anvilfolk]

This PR fixes a bunch of issues with enums.

Fixes & features

It does a general refactor of how enums are internally and in GDScript, cleaning up the code & making it more consistent across GDScript enums and native enums. It also aims to unify warnings&errors between in-editor, runtime, and export debug/release templates (native enum behavior on release templates will also require #69607).

Some examples:

class_name OuterClass

enum MyEnum { V0, V1, V2 }

class InnerClass:
	enum MyEnum { V0, V1, V2 }

It is now possible to access each enum in all possible ways:

In OuterClass, referring to:

• OuterClass.MyEnum: V1, MyEnum.V1 and OuterClass.V1, OuterClass.MyEnum.V1
• InnerClass.MyEnum: InnerClass.V1, InnerClass.MyEnum.V1 and OuterClass.InnerClass.V1, OuterClass.InnerClass.MyEnum.V1

In InnerClass, referring to:

• OuterClass.MyEnum: OuterClass.V1, OuterClass.MyEnum.V1
• InnerClass.MyEnum: V1, MyEnum.V1, InnerClass.V1, InnerClass.MyEnum.V1, OuterClass.InnerClass.V1, OuterClass.InnerClass.MyEnum.V1

For native enums, e.g. with TileSet, you can now use TileSet.TILE_SHAPE_SQUARE or TileSet.TileShape.TILE_SHAPE_SQUARE, and a warning will be issued if the native type does not have an accessed property.

All the following accesses to GDScript enums behave the the same in-editor, in debug and release exports.

Typing with enums now works properly for variables, function parameters and function returns, and comprehensive unit tests have been added to check this.

To do

• Further unify behavior between native enums and GDScript enums. ~~- Add more error unit tests to catch incompatible parameters~~
• Static check that values assigned to enum-typed variables are within bounds

Notes

Added a couple of quick extra context lookups that I believe will be superseded by https://github.com/godotengine/godot/pull/69471 and https://github.com/godotengine/godot/pull/69587. These have been noted as TODO comments in this PR, to be rechecked once all PRs are merged.