Improve syntax highlighting and outline view for Elixir (#2582)

Fixes
https://linear.app/zed-industries/issue/Z-2208/outline-view-doesnt-differentiate-between-overloaded-functions
Fixes
https://linear.app/zed-industries/issue/Z-2205/elixir-syntax-highlighting-not-working-properly-for-doc-attributes-and

This PR improves syntax highlighting and outline view in Elixir. It's
common to overload elixir functions, with many different versions of the
function for different patterns of parameters, so I updated the outline
view to show functions' parameters in Elixir. But if we showed functions
the same way in the *breadcrumbs*, it would take up too much space.

So I added a new capture in languages' `outline` queries called
`@context.extra`, which is included in the outline view, but not in
breadcrumbs.

Release Notes:

- Improved syntax highlighting of doc attributes and special macros in
Elixir
- Updated the outline view in Elixir to display function parameters, to
allow differentiating between function overloads.