Never hide backslash escapes in fontified code blocks

[jimporter]

Description

Previously, if hiding markup and using native fontification of code blocks, backslash escapes could get hidden inside code. To fix this, we just need to mark markdown-code-face as a literal face.

Related Issue

766 (see the end of the discussion).

Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] Improvement (non-breaking change which improves an existing feature)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] I have read the CONTRIBUTING.md document.
• [ ] I have updated the documentation in the README.md file if necessary.
• [x] I have added an entry to CHANGES.md.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed (using make test).

[syohex]

Thanks

[jimporter]

Thanks for merging.