Add a type for merging classes

[ajvincent]

TypeScript's documentation for class decorators makes one thing clear, at the very end of the section: any new class fields you define are not available after the decorator returns.

TypeScript's supported decorators as experimental for a while, but version 5.0 supports ECMAScript stage 3 decorators. For a while I've been wondering how to use a class decorator to build a subclass. Not having the type information is annoying.

I have a minimal fix, though.
https://github.com/ajvincent/es-membrane/blob/master/_01_stage_utilities/source/DecoratedClass.mts

With a simple test:
https://github.com/ajvincent/es-membrane/blob/master/_01_stage_utilities/spec/DecoratedClass.mts

I'm still working through the function code, but the DecoratedClass type works pretty well.

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[ajvincent]

Hm, a better name for this might be MergeClass, as we can hit this without decorators. Also, because there's already Merge and Class types in type-fest.

[ajvincent]

type MixinClass<
  AddedStatic,
  AddedInstance,
  BaseClass extends Class<unknown>,
  Arguments extends unknown[] = ConstructorParameters<BaseClass>,
> = Constructor<InstanceType<BaseClass> & AddedInstance, Arguments> & Omit<BaseClass, "prototype"> & AddedStatic;

[ajvincent]

Duplicate of #274. But I can still write a pull request.