[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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