Add Elixir LSP server configuration to default LSP clients

This adds the Elixir LSP server to the list of default clients. Elixir-ls comes with startup scripts and they should be on the PATH for this to work. I've written about a very way to setup this on Kate currently here.

I confess there is an issue with this default configuration that it assumes a UNIX system (.sh startup script). For Windows users they would need to change that to .bat.

I totally understand if my confession makes this impossible to be merged. Even I myself was not sure if this is right. So, feel free to totally ignore this and just close the MR.