[sksl][wgsl] Field access and test for vertex stage builtins

* Added support for field access expressions, excluding user-defined
  interface blocks.
* Added a test case for vertex stage builtin declarations.
* Added a NOP workaround for sk_PointSize which does not have a
  WGSL/WebGPU equivalent.

Bug: skia:13092
Change-Id: I46fd7bb0a9fe55d2e3ed2b3b72f63376c1ed4f31
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/539082
Commit-Queue: Arman Uguray <armansito@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
6 files changed