lspclient: switch to a maintained javascript language server

The javascript/typescript language server used so far has actually been
https://github.com/sourcegraph/javascript-typescript-langserver.
However, that one has now gone into unmaintained status, so switch
to the language server that is recommended there;
https://github.com/theia-ide/typescript-language-server

In this case, the server is better equipped to deal with the
javascript/typescript distinction, so we leave that up to the server.