Auto-completion with Company and Corfu (via eglot)

[monkeyjunglejuice]

The discoverability of Elixir's Libraries would be tremendously improved if autocompletion would work - afaik inf-elixir is quite important, as it is the only Elixir REPL for Emacs. Maybe get the language server autocompletions provided by eglot and "pipe" them into completion-in-region, when inside a Mix project -- meaning when inf-elixir has been started via `inf-elixir-project', which would be quite elegant, since this distinction already exists.

(I'm not quite sure if this counts as a duplicate, since that one is about getting the tab-completions from the Elixir shell, which seems to be a quite difficult, albeit more limited approach).