Add basic code action support

[lucacervello]

I added a basic code action support.

It's similar to elixir-ls which means we parse diagnostic messages in order to enable different code actions.

I tried to add an action to prepend unused variables with _ as suggested here https://github.com/elixir-lsp/elixir-ls/issues/140 but It's not working very well because I don't have the column of the variable.

Maybe there are better actions to implement ?

[lukaszsamson]

@lucacervello can you check on elixir 1.14? Since https://github.com/elixir-lang/elixir/issues/11965 is fixed compiler should return detail with structured error. Line and column should be there. If it's not let's raport it to the upstream.