[embind] Add register_type<T>(name, definition)

[mkl-ableton]

This PR adds an overload of register_type that allows registering type aliases for val-based types. With this, clients can reference a stable, named type without needing to depend on (or update alongside) its underlying definition.

E.g.,

EMSCRIPTEN_DECLARE_VAL_TYPE(MyOwnEnum);

register_type<MyOwnEnum>("MyOwnEnum", "'a' | 'b'");

This addresses the issue raised in #25145.

[FelixNumworks]

Hi ! I opened a PR to do this in another way, by adding a parameter to enum_<MyEnum>

See #25257

The result in the d.ts file would be

export type MyEnum = 'valueOne'|'valueTwo'|'valueThree'

interface EmbindModule {
  MyEnum: {valueOne: 'valueOne', valueTwo: 'valueTwo', valueThree: 'valueThree'};
};

[FelixNumworks]

I know it's a bit different as my implementation only works for enums, and not any type, but I think it adresses the problem you raised in your discussion

[mkl-ableton]

@FelixNumworks Thanks for pointing me to your PR. Exposing enums as strings is exactly my current use case. I hadn’t suggested modifying enum_ myself since I expected it might spark a bigger design discussion. Guess I should’ve just waited a week 🙂. Let’s see when/if your PR lands. I think this one makes sense regardless.