Integrate Fixit into a VSCode plugin

Integrate Fixit into a VSCode plugin (and potentially any other IDE), so developer can easily install it and use it on VSCode.

Fixit can be run automatically when developers save source file updates and show warning inline with wavy underline or lightbulb UIs.

Then developers can click on the UI or press some shortcut to apply autofix.

Some helper is provided to make this easy to implement, e.g. `lint_file` returns a list of violation reports (BaseLintRuleReport) and provides `patch` property when autofix is available.

https://github.com/Instagram/Fixit/blob/master/fixit/rule_lint_engine.py#L54