shader/validation: Add compound assignment validation

[ben-clayton]

Extend the binary operator expression validation to also validate compound assignment.

Also calculate the binary operator's resulting type, and use that to validate the result type is as expected.

Issue: #1644

---

Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• 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.

[Kangz]

I wonder if here and other places we can use the ?? operator (nullish coalescing). So it would be return elementType ?? Type.vec(lhs.width, elementType)

[ben-clayton]

That's not quite the same thing - we're returning a vector of elementType, if elementType is not null, not elementType itself.

[ben-clayton]

Thanks!