Support more ranges

[floitsch]

It would be nice if the client supported empty ranges. VSCode automatically squiggles the identifier at the position of the zero-length range. See https://github.com/Microsoft/language-server-protocol/issues/257

Even just highlighting the character at the position of the zero-length range would be a nice improvement. (Currently the diagnostics is silently dropped).

[gtache]

Ok, this isn't in the LSP spec yet but it seems it will (eventually) be added, so it would make sense to implement that.
Yeah it's not that it's willfully dropped, it's just that it's a range of size 0, so obviously the formatting isn't visible.
Thanks!

[borkdude]

Same issue here: #118