Feature Request: Add in-repo configuration file

[axelson]

I've been thinking of moving to a in-repo configuration file (e.g. .elixir-ls.conf) which would make it easier for users to configure their project. That is similar to what erlang-ls does: https://erlang-ls.github.io/configuration/

I'd imagine that the order of precedence for the preferences would go:
user home config -> repo config -> editor config
(where user home config would be a config at ~/.config/elixir-ls/elixir-ls.conf (following XDG) that would have the same available settings as the .elixir-ls.conf in the repository)

[lukaszsamson]

Closing as this is a duplicate of #60