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

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

@@ -0,0 +1,25 @@
+// IGNORE_BACKEND: JS
+// !LANGUAGE: +JsAllowInvalidCharsIdentifiersEscaping
+
+package foo
+
+class class_with_invalid_chars {
+    fun foo(): Int = 23
+}
+
+class `class@with$invalid chars` {
+    fun foo(): Int = 42
+}
+
+fun box(): String {
+    val a = class_with_invalid_chars()
+    val b = `class@with$invalid chars`()
+
+    assertEquals(true, a is class_with_invalid_chars)
+    assertEquals(true, b is `class@with$invalid chars`)
+
+    assertEquals(23, a.foo())
+    assertEquals(42, b.foo())
+
+    return "OK"
+}