Add limitations to quantity parsing logic

[hl662]

Closes #7596

• Improve tests, ensuring default parsing and formatting in core frontend's QuantityFormatter produces equivalent values between each other.
• Update the tokenization logic, adding limits and document them in scenarios where a spacer conflicts with a math operator. In particular:
  • Mathematical operations only apply when it is in between whitespace. So -2FT 6IN + 6IN is equal to -2FT-6IN + 6IN, and -2FT-6IN - 6IN is not equal to -2FT-6IN-6IN.
  • For a value like 2FT 6IN-0.5, the - sign will be treated as a spacer and not subtraction. However, the 0.5 value will use the default unit conversion provided to the parser, because it's not a part of the composite unit when that composite is made up of only 2 units - FT and IN.