Thanks a lot for your contribution! I won't be able to merge this PR in this repo, since the extension here doesn't require having rebar3 or even an Erlang installation, but I will merge it to https://github.com/yuce/vscode-erlang-next (which will become the next version of this extension) if that's OK with you.

I notice that you are implementing a language server (whatels). Does it support language server vscode?

It doesn't support vscdode language server protocol, but I've started to use work on whatels before the protocol was released to public. whatels is supposed to be able to be used from any editor, node module for it is here: https://github.com/yuce/whatels-node.

@fireflyc did you ever find or implement an extension that supports formatting for erlang files?