Add HTML aria attribute autocomplete

syntax highlighting for c++: 'value' keyword removed.

fixes the internal builds

Fixes #320

fixes #252

Make sure not each provider registers a support

Add more extensions to built-in PowerShell support

Update css at-rule support for auto-complete

fixes #210

Use auth tokens for downloading omnisharp builds from GitHub

- you can collapse/expand results and this is preserved
- the "collapse all" action enables once results arrive
- you can remove results and they dont come back at the end of the search
- we do not add the same matches over and over again to the tree, but only once per result

Fix CSS frequency capitalization