Parse function natspec and add a missing-natspec detector

[ChrisJr404]

Refs #1916. Functions now keep their natspec text so a detector can flag public/external functions that have none.

Today Function only carries a has_documentation bool the parser flips on, and the comment next to it says "To be improved". This wires it up properly. The solc parser already sees the documentation node solc emits per function, so I pull the actual text out of it (plain string on old solc, {"text": ...} on newer) and store it. has_documentation becomes a property derived from whether that text is present, so the existing caller (the natspec-generation tool) keeps working. Same treatment on the vyper side: doc_string now stores the text instead of just setting a flag.

The new missing-natspec detector (informational) walks derived contracts and reports public/external functions with no natspec. Constructors, fallback/receive, and internal/private functions are skipped to keep it quiet, and dependency code is ignored.

For tests I added test_function_comments to the parsing unit tests, which compiles a contract with a documented and an undocumented function and checks both the extracted text and the derived bool. There's also the usual e2e detector regression: a contract mixing a documented external fn, undocumented external/public fns, an internal fn, a constructor and a receive, plus the generated snapshot (only the two undocumented public/external fns get reported). Ran the detector e2e and the code-comments unit tests locally, both pass, ruff clean.

The issue also mentions events, errors and state variables. I kept this to functions so it stays a focused replacement of the existing stub. Happy to follow up on the rest if you'd like it in the same detector.

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