Add plugin for syntax error detection with Jedi

[danzvara]

This adds a plugin for accessing Jedi's syntax error detection. From v0.17.0 Jedi provides Script.get_syntax_errors to display syntax errors (they only support Syntax and Indent error right now), so I thought it would be nice to have this included in PYLS. It's my first PR here, so apologize if I forgot about something obvious, appreciate any comments/suggestions! :)

[palantirtech]

Thanks for your interest in palantir/python-language-server, @danzvara! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.