Add WGPU_MAKE_INIT_STRUCT and WGPUStringView

[kainino0x]

This is basically copied from Dawn except:

• Use NULL instead of nullptr (nullptr was added in C23)
• Put WGPU_NULLABLE in the correct place per #190 so we don't forget about it later
• Documentation! I added a few important things we did not discuss:
  • Places that accept non-nullable stringviews simply default to empty string (e.g. labels)
  • Outputs from the API always use explicit-length stringviews which may or may not be null terminated

Issue: #170 (not fixed yet, this is just the start) (dawn bug https://crbug.com/42241188)