Add option to configure a custom language server

[kkostov]

This PR enables the extension to use a custom language server.

• Added configuration option “Language Server” expecting the path to the language_server.sh of a local build of https://github.com/elixir-lsp/elixir-ls.
• The extension will not attempt to change permissions when using a custom language server.

[raulchedrese]

Thanks for contributing!

Apologies for the delay in taking a look. I left a couple comments once you get a chance to take a look at those I think we can merge this.

[kkostov]

@raulchedrese, good catch! It does indeed look much better after the fix.