[FIR] KT-58065: Support LT in K2JsIrCompiler

compiler/testData/cli/js/firLightTree.kt

@@ -0,0 +1,3 @@
+fun MutableList<String>.test() {
+    this += "Alpha"
+}

compiler/testData/cli/js/firLightTreeOn.args

@@ -0,0 +1,10 @@
+$TESTDATA_DIR$/firLightTree.kt
+-language-version
+2.0
+-Xuse-fir-lt=true
+-ir-output-dir
+$TEMP_DIR$
+-ir-output-name
+fir
+-libraries
+libraries/stdlib/js-ir/build/classes/kotlin/js/main

compiler/testData/cli/js/firLightTreeOn.out

@@ -0,0 +1,2 @@
+warning: language version 2.0 is experimental, there are no backwards compatibility guarantees for new language and library features
+OK