Improve diffs inside Elixir sigils

[crbelaus]

This pull request makes the following changes:

1. Updates the vendored tree-sitter-elixir package
2. Adds a few Elixir example files so we can compare diffs before and after the changes
3. Updates the Elixir parser to consider sigils as atom nodes

This is the result (before and after):

Notice how after the changes, the modifications inside the ~H sigil are highlighted when previously they were not. This fixes #784

[crbelaus]

Looks like updating the tree-sitter-elixir subtree has included many commits from that repository. Let me know if you prefer me to squash them into a single commit instead.

[Wilfred]

Merged in b0a4ddc, thanks for the fix and the regression test :)