Update sbt-scalajs, scalajs-library_2.13, ... to 1.9.0 (#600)

Author: scala-steward

project/plugins.sbt

@@ -12,6 +12,6 @@ addSbtPlugin("com.thoughtworks.example" % "sbt-example" % "9.1.0")
 
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.1.0")
 
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.8.0")
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.9.0")
 
 addSbtPlugin("com.thoughtworks.sbt-scala-js-map" % "sbt-scala-js-map" % "4.1.1")