Update sbt-scalajs, scalajs-compiler, ... to 1.12.0 (#634)

Co-authored-by: Seth Tisue <[email protected]>

Author: scala-steward

project/plugins.sbt

@@ -1,6 +1,6 @@
 addSbtPlugin("org.scala-lang.modules" % "sbt-scala-module" % "3.0.1")
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.2.0")
 addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "1.2.0")
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.11.0")
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.12.0")
 addSbtPlugin("org.scala-native" % "sbt-scala-native" % "0.4.9")
 addSbtPlugin("de.heikoseeberger" % "sbt-header" % "5.9.0")