[FIR] Implement support of MPP in JS CLI

compiler/testData/cli/js/firMpp.args

@@ -0,0 +1,10 @@
+$TESTDATA_DIR$/firMpp/common.kt
+$TESTDATA_DIR$/firMpp/jvm.kt
+-Xcommon-sources=$TESTDATA_DIR$/firMpp/common.kt
+-language-version
+2.0
+-ir-output-dir
+$TEMP_DIR$
+-ir-output-name
+firMpp
+-XXLanguage\:+MultiPlatformProjects

compiler/testData/cli/js/firMpp.out

@@ -0,0 +1,11 @@
+warning: ATTENTION!
+This build uses unsafe internal compiler arguments:
+
+-XXLanguage:+MultiPlatformProjects
+
+This mode is not recommended for production use,
+as no stability/compatibility guarantees are given on
+compiler or generated code. Use it at your own risk!
+
+warning: language version 2.0 is experimental, there are no backwards compatibility guarantees for new language and library features
+OK

compiler/testData/cli/js/firMpp/common.kt

@@ -0,0 +1 @@
+expect fun foo(): String

compiler/testData/cli/js/firMpp/jvm.kt

@@ -0,0 +1,5 @@
+actual fun foo() = "OK"
+
+fun test(): String {
+    return foo()
+}