Don't treat backslashes as escapes inside literal blocks

[jimporter]

Description

Backslashes get fontified as markup characters in too many situations. In particular, a backslash inside a literal block (e.g. code, math) should just be a literal backslash, not the introducer of an escape sequence.

It might be worthwhile to add some further regression tests here, but the one I did add fails without the fix, and passes with it.

Related Issue

See #766.

Type of Change

• 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

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