Upgrade to Scala.js 1.6.0.

This includes changes in the compiler and build from the following
upstream commits:

* scala-js/scala-js@c4aa18b
  Allow to specify target versions of ECMAScript > 2015.
* scala-js/scala-js@89dfc48
  Use a dedicated function for Long-to-Float conversions.

compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala

@@ -2633,7 +2633,10 @@ class JSCodeGen()(using genCtx: Context) {
               js.UnaryOp(IntToLong, intValue)
           }
         case jstpe.FloatType =>
-          js.UnaryOp(js.UnaryOp.DoubleToFloat, doubleValue)
+          if (from == jstpe.LongType)
+            js.UnaryOp(js.UnaryOp.LongToFloat, value)
+          else
+            js.UnaryOp(js.UnaryOp.DoubleToFloat, doubleValue)
         case jstpe.DoubleType =>
           doubleValue
       }

project/Build.scala

@@ -1160,7 +1160,8 @@ object Build {
             "compliantModuleInit" -> (sems.moduleInit == CheckedBehavior.Compliant),
             "strictFloats" -> sems.strictFloats,
             "productionMode" -> sems.productionMode,
-            "es2015" -> linkerConfig.esFeatures.useECMAScript2015,
+            "esVersion" -> linkerConfig.esFeatures.esVersion.edition,
+            "useECMAScript2015Semantics" -> linkerConfig.esFeatures.useECMAScript2015Semantics,
         )
       }.taskValue,
 

project/ConstantHolderGenerator.scala

@@ -35,6 +35,7 @@ object ConstantHolderGenerator {
   private final def literal(v: Any): String = v match {
     case s: String  => "raw\"\"\"" + s + "\"\"\""
     case b: Boolean => b.toString
+    case i: Int     => i.toString
     case f: File    => literal(f.getAbsolutePath)
 
     case _ =>

project/plugins.sbt

@@ -2,7 +2,7 @@
 //
 // e.g. addSbtPlugin("com.github.mpeltonen" % "sbt-idea" % "1.1.0")
 
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.5.1")
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.6.0")
 
 addSbtPlugin("org.xerial.sbt" % "sbt-sonatype" % "3.6")