rules: add co-authored-by-is-trailer

[tniessen]

This rule ensures that Co-authored-by is a trailer and does not appear at the beginning of any non-trailer lines. Violations of this rule have caused some commits to not be attributed to co-authors in the core repository.

cc @nodejs/build @nodejs/node-core-utils

Refs: nodejs/node-core-utils#602

[codecov]

Codecov Report

Merging #93 (eb590e6) into main (1eca2be) will increase coverage by 0.77%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
+ Coverage   74.88%   75.65%   +0.77%     
==========================================
  Files          18       19       +1     
  Lines         442      456      +14     
==========================================
+ Hits          331      345      +14     
  Misses        111      111              

Impacted Files	Coverage Δ
lib/rules/co-authored-by-is-trailer.js	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1eca2be...eb590e6. Read the comment docs.