Auto-install tremor-language-server

[anupdhml]

As part of the extension installation (or when a supported tremor filetype is opened), we should be able to auto-install tremor-language-server so that users don't have do it manually.

This will work better once we have binaries for all the major OSes. Can follow the approach of rust-analyzer extension, after that:

https://github.com/rust-analyzer/rust-analyzer/blob/2020-04-13/editors/code/src/main.ts#L203