Move VisualShader to its own folder, allow building export template without it

[beicause]

Implement the suggestion of godotengine/godot-proposals#10738 (comment). Disabling visual_shader makes Linux and Android template_release smaller by ~1.8 MB

[KoBeWi]

Isn't it possible to move VisualShader to its own module? 🤔

[beicause]

It is possible, but ideally the editor should not depend on modules, more work is needed to refactor the shader editor and add an abstraction layer. If #64596 gets salvaged I’d be happy to close this PR.

[KoBeWi]

Doesn't look like there is an easy way to remove VisualShader dependencies from the editor and refactoring isn't really worth it if we don't plan to support more shader types.

EDIT:
Ok, now I see that this only handles export templates. This makes it simpler, and is why the module would be less feasible.

[KoBeWi]

Needs buildsystem review, other changes look fine.

[aaronfranke]

I opened PRs #109998 and #109999 to clean up the shader editor code, and rebased PR #64596.

[aaronfranke]

PR #112100 separates out the language-specific code, and is the last cleanup before PR #64596.