feat(yarn): yarn 2.x support

bin/compile

@@ -36,7 +36,7 @@ cleanup_cache
 download_node
 install_node
 install_npm
-if [ -f "$assets_dir/yarn.lock" ]; then
+if [ -f "$assets_dir/yarn.lock" ] || [ -d "$assets_dir/.yarn" ] || [ -f "$assets_dir/.pnp.*" ]; then
   install_yarn "$heroku_dir/yarn"
 fi
 

compile

@@ -1,4 +1,4 @@
-if [ -f "$assets_dir/yarn.lock" ]; then
+if [ -f "$assets_dir/yarn.lock" ] || [ -d "$assets_dir/.yarn" ] || [ -f "$assets_dir/.pnp.*" ]; then
   yarn deploy
 else
   npm run deploy

lib/build.sh

@@ -89,7 +89,7 @@ install_npm() {
 install_yarn() {
   local dir="$1"
 
-  if [ ! $yarn_version ]; then
+  if [ ! $yarn_version ] || is_yarn2_configured; then
     echo "Downloading and installing yarn lastest..."
     local download_url="https://yarnpkg.com/latest.tar.gz"
   else
@@ -112,6 +112,10 @@ install_yarn() {
   chmod +x $dir/bin/*
   PATH=$dir/bin:$PATH
   echo "Installed yarn $(yarn --version)"
+
+  if is_yarn2_configured; then
+    yarn set version berry
+  fi
 }
 
 install_and_cache_deps() {
@@ -231,3 +235,17 @@ remove_node() {
   rm -rf $assets_dir/node_modules
   rm -rf $heroku_dir/node
 }
+
+is_yarn2_configured() {
+  if [ ! $yarn_version ]; then
+    return $(false)
+  else
+    local regex='[^0-9]*\([0-9]*\)[.]\([0-9]*\)[.]\([0-9]*\)\([0-9A-Za-z-]*\)'
+    local major_version=`echo $yarn_version | sed -e "s#$regex#\1#"`
+    if [ $major_version -ge 2 ]; then
+      return $(true)
+    else
+      return $(false)
+    fi
+  fi
+}