chore: move to sphinx_inv for inv parsing

[aslowwriter]

Issue addressed

Fixes #102

Explanation

I've moved all of the sphinx inventory parsing into it's own crate, and moved it over to winnow parsing which was way more performant (like the regex is like 200% runtime on average). Thankfully the design of that slotted into snakedown really easily!

General Checklist

• Updated tests or added new tests (if applicable)
• Branch is up to date with main
• Tests & lints hooks pass
• Updated documentation (if applicable)
• History of branch is cleaned up

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.0%. Comparing base (03bfc4b) to head (db856c5).

Additional details and impacted files

Files with missing lines	Coverage Δ
src/lib.rs	92.0% <100.0%> (+<0.1%)	⬆️

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