Macro related Go to definition support and other integrations

Opening this to track support for third party library integrations to Expert

Some libraries like Phoenix LiveView, Nx and Ash have special needs when it comes to go to definition inside their macros.

For example:
- LiveView uses the `~H` sigil, and go to definition should work there for function components or interpolated content
- Nx would benefit from smarter go to definition for calls related to `defn` and `deftransform`
- Ash could benefit from enriching the results we provide to the editor, similar to what the [Ash Studio](https://marketplace.visualstudio.com/items?itemName=ketupia.ash-studio) extension does

We need better direct integration with Expert to support libraries. The general sentiment seems to be that the current plugin architecture is lacking for this.