Pick up environment variable or binary from PATH

[eikooc]

Hi, would it be possible for this extension to pick up either an environment variable ROC_LSP_PATH or find the roc_language_server from the PATH if no path was set for roc-lang.language-server.exe.

If you agree to this, do would accept a PR for it?

[Anton-4]

find the roc_language_server from the PATH if no path was set for roc-lang.language-server.exe.

I think this is a good idea @eikooc! Feel free to make a PR, if @ivan-demchenko is busy I'll fork this repo and merge the PR there.