Switch from legacy fs extension to new files extension

[felixfbecker]

The old fs/readFile extension should be replaced by the new workspace/files, textDocument/content extension.

https://github.com/sourcegraph/language-server-protocol/blob/master/extension-files.md

[beyang]

This could yield a substantial performance boost, as you'd brought up earlier. Instead of having to do successive readDir requests, it just does one workspace/xfiles request on initialization. It might significantly simplify the code associated with mirroring the remote filesystem structure locally.