LSP: Support SymbolInformation[] response type

[lambdahands]

Some language servers (like cssls) don't respond to "textDocument/documentSymbol" with the type DocumentSymbol[], but instead SymbolInformation[].

This is a flat list of symbols in contrast to a hierarchy of symbols with children properties. ​There are other subtle differences, like the lack of a detail field, as well the range living in location.range instead of range. See here in the LSP spec for details about the two types.

This code was put together quickly, so I'm not sure how well it holds up. I found vim.lsp.utils.symbols_to_items(), but it seems to lose name information that's used to build the resulting lines.

Speaking of lines, I ran into a strange issue in which newlines were appearing in the resulting strings, causing nvim_buf_set_lines to fail. Let me know if I'm on the right track here, and thank you for your work on such an awesome plugin!

[GustavoKatel]

hi @lambdahands

I committed your sanitized code here: a562af1

it is now on the dev branch. thanks a lot!

regarding the LSP issue I think it all looks fine. So to merge this PR:

1- can you rebase from dev and use the sanitize code I pushed there?
2- make the PR as "ready for review" when done so I can giver another look :)

thanks!