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

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

@@ -0,0 +1,13 @@
+// !LANGUAGE: +JsAllowInvalidCharsIdentifiersEscaping
+
+package foo
+
+val _my_invalid_variable = 23
+val `my@invalid variable` = 42
+
+fun box(): String {
+    assertEquals(23, _my_invalid_variable)
+    assertEquals(42, `my@invalid variable`)
+
+    return "OK"
+}