Validation for address space and access mode in var decls

[alan-baker]

• Check that only storage allows an access mode (read or read_write)
• Tests no access mode parameter and a trailing comma after address space separately

Tint currently fails webgpu:shader,validation,decl,var:address_space_access_mode:address_space="*";access_mode="";trailing_comma=true (see https://crbug.com/tint/2233). webgpu:shader,validation,decl,var:address_space_access_mode:address_space="storage";access_mode="read";trailing_comma=true webgpu:shader,validation,decl,var:address_space_access_mode:address_space="storage";access_mode="read_write";trailing_comma=true

Issue: #

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Requirements for PR author:

• [x] All missing test coverage is tracked with "TODO" or .unimplemented().
• [x] New helpers are /** documented */ and new helper files are found in helper_index.txt.
• [x] Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• [x] Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• [ ] Tests are properly located in the test tree.
• [ ] Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• [ ] Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• [ ] Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.