feat: Add file-types command

[Pringled]

This PR adds a semble file-types command, which prints the file extensions indexed for each content type as JSON. --content narrows the output to specific types. This resolves #292.

Example:

$ semble file-types --content docs
{"docs": [".adoc", ".md", ".rst", ...]}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/semble/cli.py	100.00% <100.00%> (ø)
src/semble/version.py	100.00% <100.00%> (ø)

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[greptile-apps]

Confidence Score: 5/5

[Low impact] The PR appears safe to merge.

_{Reviews (2) · Last reviewed commit: "Assert representative extensions in file..." · Reviewed by Greptile}

[stephantul]

smort