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

js/js.translator/testData/box/escapedIdentifiers/externalEscapedClassFields.js

@@ -0,0 +1,15 @@
+function A() {
+    this["@invalid @ val@"] = 23
+    this["--invalid-var"] = "A: before"
+}
+
+A.prototype["get something$weird"] = function() {
+    return "something weird"
+}
+
+A["static val"] = 42
+A["static var"] = "Companion: before"
+
+A["get 🦄"] = function() {
+    return "🦄"
+}