Implement a Formatter for Tree-Sitter Grammar Tests in Darklang

We tried to plug tree-sitter-tests-formatter in our repository, to format tree-sitter-darklang/test/corpus test files, but decided against it due to the 1.5 GB ( for Rust) it would add to the Dockerfile

We want to write a formatter for our Tree-sitter grammar tests in Darklang

Notes:

• Handle nested directories : not all test files are in tree-sitter-darklang/test/corpus; some are in nested directories, e.g.tree-sitter-darklang/test/corpus/exhaustive/exprs
• maintain a consistent number of # symbols at the beginning of each test header
• trim test names
• keep the test case as is (no trimming no formatting)
• have a triple hyphen (---) separator between the test case and the generated tree
• spacing rules: two newlines after each test block, one after each title, test case, and after the triple hyphen separator
• format the S-expression based on the following example

Example of a formatted test:

==================
boolean - true
==================

true

---

(source_file 
  (expression 
    (simple_expression 
      (bool_literal)
    )
  )
)

Misc:

• Run the script using the cli?
• Figure out a way to run it in CI

Start with the test files under tree-sitter-darklang/test/corpus, including nested directories such as exhaustive/exprs, and compare them with the formatted example in the issue. Define the formatter's CLI and CI entry points, then verify that headers, spacing, separators, test-case text, and S-expressions follow the stated rules across all corpus files.