Make use of diagnostics methods in LS

[MischaPanch]

Should be easy and really help, get diagnostics after every edit. Don't accept edits that create syntax errors

Maybe make tool "get problems in code" like in IDEs.

Since this is central functionality for LS, should not require much/any hacking

[dsyme]

@MischaPanch I'm kind of interested in adding this if #660 lands OK to your liking.