feat(changelog): add opt-in Markdown formatting for the output

[ChrisJr404]

Closes #678. Adds an opt-in [changelog] format = true that runs the rendered changelog through a Markdown formatter, so you can get tidy output without fighting the template with {%- and trim everywhere.

It's off by default and the output is byte-for-byte what it is today unless you turn it on. When enabled it only kicks in for Markdown output (stdout or a .md file, detected by extension) — point --output at something else and it's skipped with a warning.

On the crate choice: the issue floated prettify-markdown, but as the last commenter noted that one's still alpha and doesn't cover much of Markdown. I went with a pulldown-cmark + pulldown-cmark-to-cmark round-trip instead — both are actively maintained and GFM-complete. The formatting is deliberately conservative: it keeps - as the bullet marker (matching the default templates, so existing changelogs don't churn - -> *), enables tables/strikethrough/task lists/footnotes so those survive, and doesn't reflow text or rewrite links.

One tradeoff worth flagging: the round-trip escapes bare bracket text, so a heading like ## [unreleased] or a first release's ## [1.0.0] with no compare URL comes out as ## \[unreleased\]. That's standard CommonMark formatter behavior (prettier and mdformat do the same — bare [text] is an ambiguous undefined shortcut link) and headings that are real links like ## [1.0.0](…/compare/…) are untouched. Didn't want to hack around it silently; happy to add an option to disable bracket escaping if you'd rather.

Kept it in git-cliff-core as a small markdown helper so it's unit-testable. Tests: the helper normalizes a messy-but-valid changelog and is idempotent, a clean one round-trips unchanged, and at the generate() level format = true produces the formatted output while leaving it unset is identical to before. cargo test -p git-cliff-core is green (75 passed), cargo +nightly fmt --check and cargo clippy are clean. Also smoke-tested the binary with GIT_CLIFF__CHANGELOG__FORMAT=true against this repo.

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[ChrisJr404]

Heads up on the two red checks, both look unrelated to this change. The Test suite job runs every test green and only fails on the codecov upload step (gpg: Can't check signature: No public key), which is the same failure that's currently red on main. The Links check is a lychee timeout on the contributor-covenant.org URL in CODE_OF_CONDUCT.md, a file this PR doesn't touch. A re-run should clear both whenever you get a chance.