Support for Elixir/Erlang and the Phoenix framework

### 🤔 What's the problem you're trying to solve?

The VS Code extension landing page told me to open an issue if my language is unsupported, so I'm doing so now. Elixir and Erlang with any added QOL improvements for folks using the Phoenix framework would be much appreciated.

### ✨ What's your proposed solution?

Adding language support for Elixir and Erlang

### ⛏ Have you considered any alternatives or workarounds?

I have not

### 📚 Any additional context?

I'm trying to do stuff in Phoenix and would love to be able to use this framework

Start from the VS Code extension landing page and inspect how currently supported languages are registered. Clarify with maintainers what Elixir, Erlang, and Phoenix support should cover; done means the agreed language support and Phoenix quality-of-life improvements are implemented and documented.

Written by the indexing model from the issue text.