Exclude vendored coverage-report CSS from stylelint

coverage/html/_css/*.css is third-party CSS bundled into the PHPUnit
coverage HTML report, not plugin source. It wasn't covered by the
default @wordpress/scripts ignore file (which only skips build/ and
vendor/), so it was inflating lint output. build/, dist/, and vendor/
are added explicitly too so exclusion doesn't depend on the bundled
default ignore file.

Author: pattonwebz

.stylelintignore

@@ -0,0 +1,5 @@
+build
+dist
+vendor
+coverage
+node_modules