# Description
Old code was comparing remaining positional arguments with total number
of arguments, where it should've compared remaining positional with
with remaining arguments of any kind. This means that if a function was
given too few arguments, `calculate_end_span` would believe that it
actually had too many arguments, since after parsing the first few
arguments, the number of remaining arguments needed were fewer than the
*total* number of arguments, of which we had used several.

Fixes #9072
Fixes: #13930
Fixes: #12069
Fixes: #8385

Extracted from #10381

## Bonus

It also improves the error handling on missing positional arguments
before keywords (no longer crashing since #9851). Instead of just giving
the keyword to the parser for the missing positional, we give an
explicit error about a missing positional argument. I would like better
descriptions than "missing var_name" though, but I'm not sure if that's
available without

Old error
```
Error: nu::parser::parse_mismatch

  × Parse mismatch during operation.
   ╭─[entry #1:1:1]
 1 │ let = if foo
   ·     ┬
   ·     ╰── expected valid variable name
   ╰────
```

New error
```
Error: nu::parser::missing_positional

  × Missing required positional argument.
   ╭─[entry #18:1:1]
 1 │ let = foo
   ·    ┬
   ·    ╰── missing var_name
   ╰────
  help: Usage: let <var_name> = <initial_value>
```

# User-Facing Changes
The program `alias = = =` is no longer accepted by the parser