Simplify some type constants

[dj2]

This CL adds kAllScalarsAndVec and a
kConvertableToFloatScalarsAndVec helper types which allow removing a couple of the combined type helpers.

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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.

[zoddicus]

overall I like the change, but apparently cannot avoid bikeshedding.

Having now seen the names in situ, how do you feel about changing '*ScalarsAndVec' to '*ScalarsAndVectors' and onlying using the short form when refering to just vectors of a specific dimention? So kAllScalarsAndVec would become kAllScalarsAndVectors, etc.

The existing code is somewhat incosistent on the use of Vec vs Vector(s). I think I would prefer moving to using the full Vectors when talking about vectors in general and the short form, VecN, when talking about a specific size (Since vecN is the syntax in WGSL)

[dj2]

Done.