Add Elixir and Zig tree-sitter support

[thegdsks]

Two more languages worth adding. Both have mature tree-sitter grammars available.

Same process as other languages:

1. Add grammar dependency
2. Write export/import extraction queries
3. Add test fixtures
4. Update README

Elixir is interesting because module exports work differently (defdelegate, use, import). Zig is straightforward but the pub keyword scoping needs careful handling.