feat(Escaped Identifiers): add ability to use any symbol wrapped in back ticks.

js/js.translator/testData/box/escapedIdentifiers/classLikeMemberClassMangling.kt

@@ -0,0 +1,23 @@
+// IGNORE_BACKEND: JS
+// !LANGUAGE: +JsAllowInvalidCharsIdentifiersEscaping
+
+package foo
+
+@JsExport()
+class A {
+    class `$invalid inner` {}
+}
+
+class B {
+    class `$invalid inner` {}
+}
+
+fun box(): String {
+    // DCE preventing
+    val b = B()
+
+    assertEquals("function", js("typeof A['\$invalid inner']"))
+    assertEquals(js("undefined"), js("B['\$invalid inner']"))
+
+    return "OK"
+}