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

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

@@ -0,0 +1,20 @@
+// IGNORE_BACKEND: JS
+// !LANGUAGE: +JsAllowInvalidCharsIdentifiersEscaping
+
+package foo
+
+fun _my_fn(a: Int): Int { return a }
+fun `my fn`(): Int { return 42 }
+
+fun box(): String {
+    val fn1 = ::_my_fn
+    val fn2 = ::`my fn`
+
+    assertEquals("_my_fn", fn1.name)
+    assertEquals("my fn", fn2.name)
+
+    assertEquals(23, _my_fn(23))
+    assertEquals(42, `my fn`())
+
+    return "OK"
+}