[WIP][RFC] Move to native flow LSP

[jbrown215]

Flow natively supports the LSP now, so we can start to move this plugin over to the native Flow LSP.

I have 0 experience with LSP or VSCode extensions, so I'm looking for a lot of feedback here from people who know this technology better than I do.

I wanted to get the ball rolling on this quickly, since nuclide recently announced that it is retiring. If you feel like you can get this done quicker than I can, feel free to take over this PR.

Relevant configuration parsing in flow is here: https://github.com/facebook/flow/blob/master/hack/utils/lsp_fmt.ml#L799

I've tested by running the plugin via the instructions in CONTRIBUTING.md and writing flow with errors to see if the LSP catches them. It does. Here is my flow server log, which you can see is using the LSP:
log.txt

[thymikee]

This PR implements this change already: #295 plus some other sweet changes.

I suggest closing this one and focus on the one I just mentioned, maybe leave some feedback :)

[jbrown215]

Ah! I didn't realize it also moves the LSP to use flow's built-in support. super exciting!