[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/multiplatform/contracts/output.txt

@@ -9,3 +9,4 @@ Output:
 -- JS --
 Exit code: OK
 Output:
+warning: legacy compiler is deprecated. Please migrate onto IR.

compiler/testData/multiplatform/funInterfaces/output.txt

@@ -9,3 +9,4 @@ Output:
 -- JS --
 Exit code: OK
 Output:
+warning: legacy compiler is deprecated. Please migrate onto IR.

compiler/testData/multiplatform/inlineClasses/output.txt

@@ -21,6 +21,7 @@ actual class Foo2 actual constructor(actual val y: String)
 -- JS --
 Exit code: COMPILATION_ERROR
 Output:
+warning: legacy compiler is deprecated. Please migrate onto IR.
 compiler/testData/multiplatform/inlineClasses/js.kt:5:1: error: actual class 'Foo2' has no corresponding expected declaration
 The following declaration is incompatible because modifiers are different (companion, inner, inline):
     public final expect inline class Foo2

compiler/testData/multiplatform/jsNameClash/output.txt

@@ -5,3 +5,4 @@ Output:
 -- JS --
 Exit code: OK
 Output:
+warning: legacy compiler is deprecated. Please migrate onto IR.

compiler/testData/multiplatform/optionalExpectation/output.txt

@@ -9,4 +9,4 @@ Output:
 -- JS --
 Exit code: OK
 Output:
-
+warning: legacy compiler is deprecated. Please migrate onto IR.

compiler/testData/multiplatform/simple/output.txt

@@ -9,3 +9,4 @@ Output:
 -- JS --
 Exit code: OK
 Output:
+warning: legacy compiler is deprecated. Please migrate onto IR.