Allow escaped hyphens in character classes ([\-])

As mentioned in https://github.com/goodmami/pe/issues/59#issuecomment-3186903306, escaping a hyphen is the recommended way to get a literal `-` in a character class but it is not actually allowed in Pe's PEG metasyntax:

```pycon
>>> pe.match(r'[a\-]', "-")
Traceback (most recent call last):
[...]
pe._errors.ParseError:
line 0, character 3
[a\-]
^
ParseError: `[tnvfr"'\[\]\\\\]`, `[0-7]`, `x`, `u`, `U`, `U`

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
[...]
pe._errors.GrammarError: invalid grammar
```

This should be allowed. Furthermore, it's probably time to have separate escaping/unescaping rules for character classes and regular strings.