Type-safe API to query Godot features

We already have the [`GdextBuild`](https://godot-rust.github.io/docs/gdext/master/godot/init/struct.GdextBuild.html) class which gives access to API and runtime versions.

There are more capabilities that can be queried, e.g. using [`OS::has_feature()`](https://docs.godotengine.org/en/stable/classes/class_os.html#class-os-method-has-feature).

The [feature tags](https://docs.godotengine.org/en/stable/tutorials/export/feature_tags.html) are however stringly typed, and it would be nice to have a type-safe API. Instead of a single enum grouping all values, we could categorize features into multiple specific enums. The docs contain a bullet list:

> Godot has a special system to tag availability of features. Each feature is represented as a string, which can refer to many of the following:
> - Platform name.
> - Platform architecture (64-bit or 32-bit, x86 or ARM).
> - Platform type (desktop, mobile, Web).
> - Supported texture compression algorithms on the platform.
> - Whether a build is debug or release (debug includes the editor).
> - Whether the project is running from the editor or a "standalone" binary.
> - Many more things.

We would need to make sure that enum variants are truly exclusive, otherwise we'd need to model things differently. Knowing the exact semantics may require some research.