fix: exclude lib/ source from spell checker

TypeScript source has domain terms (jpeg, grayscale, srgb, etc.)
that aren't worth spell-checking. Only prose docs matter.

Author: lilith

.github/actions/spell-check/excludes.txt

@@ -18,6 +18,7 @@ yarn-1.19.0.js$
 \.node$
 (?:^|/)node_modules/
 (?:^|/)dist/
+(?:^|/)lib/
 (?:^|/)__snapshots__/
 ^\.github/workflows/
 ^\.npmignore$