wgsl: Access mode restrictions

[dj2]

For both variable declarations and pointer types:

• Storage address space no address mode (defaults to read)
• Storage address space with read mode accepted
• Storage address space with read_write mode accepted
• Storage address space with write mode rejected
• Other address spaces {function, private, workgroup, uniform, storage}
  • read
  • write
  • read_write

4.5.1 Access Mode Defaults

Edits: Testing the pointer type spelling has moved to #2832

[dneto0]

Fixed by #2824