[Gradle, JS] IR by default and report warnings for using legacy compiler

- just legacy - report warning about deprecation
- both - report warning about deprecation of legacy
- no compiler explicitly chosen - error about explicit setting compiler

warn from cli legacy compiler

nowarn flag - kotlin.js.compiler.nowarn

KT-42326
KT-53074

compiler/testData/integration/ant/js/sourceMap/build.log.expected

@@ -3,6 +3,7 @@ Buildfile: [TestData]/build.xml
 
 build:
 [kotlin2js] Compiling [[TestData]/root1] => [[Temp]/out.js]
+[kotlin2js] warning: legacy compiler is deprecated. Please migrate onto IR.
 
 BUILD SUCCESSFUL
 Total time: [time]