Add support for HEEx templates in Elixir, fix a bug in handling nested language injections (#2603)

Closes https://linear.app/zed-industries/issue/Z-2211/heex-support

Release Notes:

- Added support for Elixir HEEx templates.
- Fixed a bug that caused incorrect syntax highlighting in ERB templates
([#1619](https://github.com/zed-industries/community/issues/1619)).