This PR decouples the LS from direct file system access by implementing the proposals for workspace/files and textDocument/content under workspace/xfiles and textDocument/xcontent. The requests are only used when the client expressed support for them through ClientCapabilities, otherwise direct FS access is used.
This turns document content retrieval and recursive file search into async operations.
In turn, all server handlers can now operate async by returning a promise.
Ivan Bozhanov
Felix Becker
Michal Niewrzal