Full-system automated integration tests

[dgreisen]

Once #218 is merged, we can run pygls and vscode entirely in the browser. This means we can use off-the-shelf browser testing frameworks to perform automated integration/acceptance testing, in addition to the existing tests that just test the api interface for compliance with the Language Server Protocol.

It wold be worth seeing if any other projects have implemented something like this already, and if not turning this into another project that could be used by any server written in any language as long as it can run in the browser?