Fix panic on too few arguments for custom function (#10395)

# 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: https://github.com/nushell/nushell/issues/13930
Fixes: https://github.com/nushell/nushell/issues/12069
Fixes: https://github.com/nushell/nushell/issues/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

tests/repl/test_parser.rs

@@ -189,7 +189,31 @@ fn assignment_with_no_var() -> TestResult {
         "mut = 'foo' | $in; $x | describe",
     ];
 
-    let expected = "valid variable";
+    let expecteds = [
+        "missing var_name",
+        "missing var_name",
+        "missing const_name",
+        "missing var_name",
+        "missing var_name",
+    ];
+
+    for (case, expected) in std::iter::zip(cases, expecteds) {
+        fail_test(case, expected)?;
+    }
+
+    Ok(())
+}
+
+#[test]
+fn too_few_arguments() -> TestResult {
+    // Test for https://github.com/nushell/nushell/issues/9072
+    let cases = [
+        "def a [b: bool, c: bool, d: float, e: float, f: float] {}; a true true 1 1",
+        "def a [b: bool, c: bool, d: float, e: float, f: float, g: float] {}; a true true 1 1",
+        "def a [b: bool, c: bool, d: float, e: float, f: float, g: float, h: float] {}; a true true 1 1",
+    ];
+
+    let expected = "missing f";
 
     for case in cases {
         fail_test(case, expected)?;