fix: update extract meta to work with new default feature in upstream

[JosiahParry]

Closes #482

This changes the regex to exclude #[extendr(default = "...")] from the extract_meta() function allowing the knitr engine to continue working.

[JosiahParry]

related extendr/extendr#952

[CGMossa]

LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.34%. Comparing base (f9f0c8e) to head (c7f90c9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
R/find_exports.R	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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