[ffigen] Add setters for dart-style enum members for structs

[jacksonrl]

Fixes #2784. Until now I was (unknowingly) using an old version that already supported this:

(old bindings)

final class WGPUVertexAttribute extends ffi.Struct {
  external ffi.Pointer<WGPUChainedStruct> nextInChain;

  @ffi.Int32()
  external int format;

  @ffi.Uint64()
  external int offset;

  @ffi.Uint32()
  external int shaderLocation;
}

This adds support for directly setting an enum without having to use enumAsInt.

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[github-actions]

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[coveralls]

coverage: 91.674% (-0.1%) from 91.818%
when pulling ff12396 on jacksonrl:enum-setters-clean
into 5765044 on dart-lang:main.

[coveralls]

coverage: 91.819% (+0.001%) from 91.818%
when pulling bb3b4d8 on jacksonrl:enum-setters-clean
into 5765044 on dart-lang:main.

[liamappelbe]

Looks good to me. You just need to update a few more bindings to fix the test failures.

[jacksonrl]

@liamappelbe it should be fixed now.

[liamappelbe]

Thanks!