prevent parser from parsing variables as units

[merelymyself]

Description

Resolves #11274.

~/CodingProjects/nushell> let day = 2; echo 0..<$day
╭───┬───╮
│ 0 │ 0 │
│ 1 │ 1 │
╰───┴───╯
~/CodingProjects/nushell> let kb = "jan"; echo 0..$kb 
Error: nu::shell::type_mismatch

  × Type mismatch during operation.
   ╭─[entry #1:1:22]
 1 │ let kb = "jan"; echo 0..$kb
   ·                      ┬─┬─┬─
   ·                      │ │ ╰── string
   ·                      │ ╰── type mismatch for operator
   ·                      ╰── int
   ╰────

User-Facing Changes

Tests + Formatting

Relevant test added 🆙

After Submitting

[sholderbach]

Thanks for the fix and test! You are really chipping away at our bug backlog.

[crides]

This doesn't fully resolve #11274.

Things like 0..$d is fine, but things like 0..$ab (where what-could-be-the-unit starts some chars after the $). It's easy to see why from the diff though... the lhs of the unit should really just be one of:

1. number literals (ints, floats)
2. variables ($ followed by identifier/cell path)
3. parenthesized expressions (and let the evaluator deal with the incorrect type)

[sholderbach]

Good point that we can rule out more for the lhs in parse_unit_value but that should be limited to just a numeric literal based on what our unit-literals support.

I am going to reopen #11274, it seems we have more to figure out around how the range expression parsing works or how the different literal/expression parses interact in their precedence.