Improves detection of whether tab-completion is in a string and supresses Jedi. Refs #10926 and #11530 Jedi results swamp file_matches and dict_key_matches in tab-completion, which is a real nuisance. The logic in the jedi completor tried to catch cases where it was "in a string", but that logic only looked at the previous character, and was a little fragile, breaking in lots of cases such as: './<tab> "mypath/<tab> etc. This seems a bit more robust in that it searchs for the first token in the current parser tree and checks if its value starts with ' or ". This detection of "in a string" then turns of jedi and returns some sanity to the set of matches.