Some more active pattern error message improvements + tests

[brianrourkeboll]

Description

Followup to #16846 (thanks for that, @Tangent-90!).

• Don't imply that a pattern argument is needed when it may (and should) be omitted.

  E.g., for

  let (|P|) (expr1 : int) = P
  let expr2 = 2
  match 1 with P expr2 pat -> ()

  the error was originally

  error FS0001: This expression was expected to have type 'int -> 'a' but here has type 'unit'
  

  and after Add error tcActivePatternArgumentCountNotMatch #16846 became

  error FS3868: This active pattern expects exactly one pattern argument, e.g., 'P pat'.
  

  which was technically correct but encouraged adding an explicit pattern for unit, which is normally omitted.

  The error is now instead

  error FS3868: This active pattern does not expect any arguments, i.e., it should be used like 'P' instead of 'P x'.
  

• A typar with (most kinds of) constraints cannot be unit. Knowing this lets us use the improved error messages in more scenarios.

• Add more comprehensive tests for active pattern arg count error messages. They're not truly comprehensive — there are more fun combinations to try, like val (|P|) : int -> bool (as opposed to val (|P|_|) : int -> bool), val (|P|_|) : int -> bool option, etc., etc.

  

Checklist

• Test cases added.
• Release notes entry updated.

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/8.0.400.md

[psfinaki]

Amazing work and great tests, thanks!