Fix escape() not escaping backslash characters

[bysiber]

The escape() method is meant to escape special characters in a filename so it can be safely used as a gitignore pattern. Currently it escapes []!*#? but misses backslash (\), which is the escape character in gitignore patterns.

This means escape() doesn't produce a round-trippable result for filenames containing backslashes:

from pathspec import PathSpec
from pathspec.patterns.gitignore.base import _GitIgnoreBasePattern

filename = "foo\\bar"

escaped = _GitIgnoreBasePattern.escape(filename)
# Returns 'foo\\bar' — unchanged, backslash NOT escaped

spec = PathSpec.from_lines('gitwildmatch', [escaped])
spec.match_file(filename)
# Returns False — the pattern doesn't match the original filename!
# The parser consumed the backslash as an escape for 'b',
# so the pattern actually matches 'foobar' instead of 'foo\bar'

The fix adds \\ to the set of escaped characters, so escape('foo\\bar') correctly produces 'foo\\\\bar', which the parser then interprets as a literal backslash.

[cpburnz]

Thanks for finding and fixing this bug.