bug: skip_files is not OS-agnostic

[nfelt14]

Description of the bug

The skip_files option accepts literal file paths, but when the file is checked it uses OS-specific file path separators, which means each file needs to be specified twice to work on Windows/Unix operating systems.

To Reproduce

1. Set up mkdocs with plugin on Windows operating system
2. Add a file path to the skip_files option using Unix-style file path separators

      plugins:
        - spellcheck:
            skip_files:
              - path/with/unix/seps/file.txt

3. Run mkdocs and observe how the file is not skipped

Expected behavior

The skip_files option should accept file paths using Unix-style file path separators and work on both Windows and Unix operating systems.

Environment information

$ python -m mkdocs_spellcheck.debug
- __System__: Windows-10-10.0.19045-SP0
- __Python__: cpython 3.11.8
- __Environment variables__:
- __Installed packages__:
  - `mkdocs-spellcheck` v1.0.3

[pawamoy]

Good catch, thanks for the report! Should be an easy fix. Will get to it asap, PRs welcome in the meantime :)

[nfelt14]

@pawamoy I was able to get a PR up for this.

[pawamoy]

Just to confirm, as it wasn't clear to me in the PR: is this fixed with your changes?

[nfelt14]

My PR did fix this issue as a byproduct of the new feature.

[pawamoy]

Ok great!