Add Kotlin and Dart tree-sitter support

[thegdsks]

Stacklit currently parses 11 languages. Kotlin and Dart are the two most requested additions.

What needs to happen for each language:

1. Add the tree-sitter grammar dependency
2. Write queries for extracting exports (functions, classes, types)
3. Write queries for extracting imports
4. Add test fixtures with a small sample project
5. Update the language list in README

The existing language implementations in the parser package are good references. Each language is roughly 50-100 lines of query definitions.

Good first contribution if you know either language.