Take advantage of parsimonious 0.6's shorter imports.
Rename TrigramTreeVisitor to SubstringTreeVisitor, which is more accurate now that we aren't breaking down substrings into trigrams.
Implement JsRegexVisitor, which translates from DXR-flavored regexes to JS ones.

* Rename BACKSLASH_SPECIAL_CHARS to BACKSLASH_METAS to distinguish them semantically from SubstringTreeVisitor.backslash_specials.
* Break literal_class_char into its own rule so we could visit it separately in the JsRegexVisitor and turn it into text.
* Factor out visit_class_item; it's just a lift_child.
* Tersen visit_class_contents.
Add PythonRegexVisitor, for app-side highlighting.