[JS IR] Support per-file mode and ES modules

js/js.translator/testData/box/esModules/incremental/jsModule.kt

@@ -0,0 +1,41 @@
+// DONT_TARGET_EXACT_BACKEND: JS
+// ES_MODULES
+
+// MODULE: lib
+// FILE: a.kt
+@file:JsModule("./foo.mjs")
+
+external fun fooF(): String
+
+// FILE: b.kt
+@file:JsModule("./bar.mjs")
+
+external fun barF(): String
+
+// FILE: c.kt
+// RECOMPILE
+fun dummyF() = "dummy"
+
+// MODULE: main(lib)
+// FILE: main.kt
+
+fun box(): String {
+    val foo = fooF()
+    if (foo != "foo") return "fail1: $foo"
+
+    val bar = barF()
+    if (bar != "bar") return "fail2: $bar"
+
+    return "OK"
+}
+
+
+// FILE: foo.mjs
+export function fooF() {
+    return "foo";
+}
+
+// FILE: bar.mjs
+export function barF() {
+    return "bar";
+}