build: make backwards-compatible build/ directory

Author: shawnbot

script/postpublish

@@ -13,3 +13,5 @@ if [[ -f $file ]]; then
     cat $file | xargs rm -rfv
     rm $file
 fi
+
+rm -rf build/

script/prepublish

@@ -8,6 +8,12 @@ npx action-status --context="$PUBLISH_STATUS_CONTEXT" \
 # generate the build directory
 npm run dist
 
+# TODO: remove this in v13
+mkdir -p build
+cp dist/primer.css build/build.css
+cp dist/stats/primer.json build/build.json
+echo "module.exports = {cssstats: require('./build.json')}" > build/index.js
+
 files=$(git ls-files src | sed -e 's#^src/##' | sed -e 's#/.*$##' | sort -u)
 echo $files > publish-files.log
 cd src