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

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

@@ -0,0 +1,23 @@
+// IGNORE_BACKEND: JS
+// !LANGUAGE: +JsAllowInvalidCharsIdentifiersEscaping
+
+package foo
+
+external fun `@get something-invalid`(): String = definedExternally
+
+external var `some+value`: Int
+    get() = definedExternally
+    set(a: Int) = definedExternally
+
+external object `+some+object%:` {
+    val foo: String = definedExternally
+}
+
+fun box(): String {
+    val a = js("2")
+    assertEquals(42, `some+value`)
+    assertEquals("%%++%%", `+some+object%:`.foo)
+    assertEquals("something invalid", `@get something-invalid`())
+
+    return "OK"
+}