Add ParsePattern and ParsePathPattern to report why a pattern is invalid

[loderunner]

Today ValidatePattern returns a bool and Match/Glob return a bare ErrBadPattern, with no details of what went wrong, leaving the user guessing how to fix it.

This adds ParsePattern and ParsePathPattern, which return an error with the reason and byte offset:

err := doublestar.ParsePattern("ab{c,d}[")
// syntax error in pattern: unclosed character class starting at offset 7
errors.Is(err, doublestar.ErrBadPattern) // true

Backwards compatibility

• ValidatePattern/ValidatePathPattern keep their bool signatures
• Match/PathMatch/Glob still return the plain ErrBadPattern