fix: path completion in Windows using backward slash

[marovira]

Fixes #1151 by doing the following:

1. Ensure that \ is allowed as a trigger character.
2. Fix the path regex to handle Windows correctly.

The implementation for the path regex was taken directly from async_path. I've already tested this on Windows and can confirm that it correctly handles both C:/ and C:\ paths.

[Saghen]

Thanks!