Update sbt-scalajs-bundler to 0.21.0 (#170)

Author: scala-steward

project/plugins.sbt

@@ -12,7 +12,7 @@ addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.8.0")
 
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.1.0")
 
-addSbtPlugin("ch.epfl.scala" % "sbt-scalajs-bundler" % "0.20.0")
+addSbtPlugin("ch.epfl.scala" % "sbt-scalajs-bundler" % "0.21.0")
 
 addSbtPlugin("com.thoughtworks.sbt-scala-js-map" % "sbt-scala-js-map" % "4.1.1")