fix: remove inaccurate and unnecessary regexp used by unknown-options-as-args

[shadowspawn]

isUnknownOption contained three similar regular expressions about characters after a flag. After investigation, and consistent with the examples in the comments, I believe they are related to short option group processing (so the -+ at start of pattern is too permissive). But in fact they are not needed at all, because the cases are covered by hasAllShortFlags()!

      // e.g. '-a-'
      const flagEndingInHyphen = /^-+([^=]+?)-$/
      // e.g. '-abc123'
      const flagEndingInDigits = /^-+([^=]+?\d+)$/
      // e.g. '-a/usr/local'
      const flagEndingInNonWordCharacters = /^-+([^=]+?)\W+.*$/

After deleting the expressions all the existing tests still pass. Also fixes #501. I have copied across the tests from #502 and added @Xunnamius as a co-author on this PR.

Resolves #501
Closes #502