Support WGPUStringView in non-struct input arguments

- Add WGPUStringView struct. In C++ there is both wgpu::StringView and
  wgpu::NullableStringView to differentiate different types of strings.
  These structs appear identically in C.
- Add support for serializing / deserializing WGPUStringView in the wire,
  with tests.
- Update methods with input args to take WGPUStringView in addition to
  the legacy const char* path.
  Methods with WGPUStringView are suffixed with "2", but overloaded
  without the suffix in the C++ API.
- wgpu::StringView may be implicitly constructed from std::string_view,
  const char* to ease use in C++. Add tests for
  these constructors. wgpu::NullableStringView may be constructed from
  nullptr and std::nullopt
- Add tests for passing wgpu::StringView or passing it via implicit
  conversion into the API.
- WGPUStringView is passed into Dawn as std::string_view or as a
  std::optional<std::string_view> depending on whether the arg is
  nullable or not. This simplifies usage in Dawn and makes it more
  type safe.

Bug: 42241188
Change-Id: Ia3dc6ef5d55f382dc8d132d3f997bb84c08cda5b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/198795
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>