[fix] server side error: escape backslashes in the query highlight_content

Any backslash escapes in the replacement are processed [1], backslashes should
be escaped [2].

[1] https://docs.python.org/3/library/re.html#re.sub
[2] https://docs.python.org/3/library/re.html#re.escape

closes:
- https://github.com/searxng/searxng/issues/2256
- https://github.com/searxng/searxng/issues/2250