Add missing numeric trait implementations to Val

[alice-i-cecile]

What problem does this solve or what need does it fill?

bevy_ui's Val type only has implementations for Add and AddAssign with floats.

What solution would you like?

Add:

1. Subtraction, multiplication and division with floats.
2. All four of these operations with Val itself.

[alice-i-cecile]

Closed in favor of #5574.