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)

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