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

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

@@ -0,0 +1,20 @@
+// IGNORE_BACKEND: JS
+// RUN_PLAIN_BOX_FUNCTION
+// INFER_MAIN_MODULE
+// !LANGUAGE: +JsAllowInvalidCharsIdentifiersEscaping
+
+// MODULE: export_invalid_name_variable
+// FILE: lib.kt
+
+@JsExport
+val `my@invalid variable` = 42
+
+// FILE: test.js
+function box() {
+    var variableValue = this["export_invalid_name_variable"]["my@invalid variable"]
+
+    if (variableValue !== 42)
+        return "false: expect exproted 'my@invalid variable' to be 42 but it equals " + variableValue
+
+    return "OK"
+}