chore: keep vitest.config.js and .github out of the npm tarball

vitest.config.js requires vitest, a devDependency consumers don't have.
.github already shipped before this PR; the new test workflow would have
joined it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: marceloprerender

.npmignore

@@ -1,2 +1,4 @@
 test
 .travis.yml
+vitest.config.js
+.github