Check for consistency of fixed-point precision in operations

[ISSOtm]

Goal: print a warning when doing any of these

• [ ] 1.0 + 1
• [ ] 1.0 * 2.0
• [ ] MUL(1.0q8, 2.0q8, 16)
  • [ ] Check that we still warn if q8 and/or , 16 are omitted (with the appropriate OPT Q setting, of course).
• [ ] 1.0q8 + 2.0q16
• [ ] Warn if any fixed-point value is passed to RGBLINK, as it lacks precision info (and I don't think it's worth adding, fixed-point ops are clearly meant to be kept at compile time).

We may want that warning to have levels, since e.g. someone may be intentionally multiplying fixed-point numbers together and correctly dealing with the resulting precision change.

Bikeshed points:

• How many levels to have?
• What level to assign to each case?
• Which levels should be default, assigned to -Wall, assigned to -Wextra?

(Originally brought up in https://github.com/gbdev/rgbds/pull/1455#discussion_r1706841243)