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

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

@@ -0,0 +1,39 @@
+// IGNORE_BACKEND: JS
+// !LANGUAGE: +JsAllowInvalidCharsIdentifiersEscaping
+
+package foo
+
+external class A {
+    val `@invalid @ val@`: Int = definedExternally
+    var `--invalid-var`: String = definedExternally
+
+    fun `get something$weird`(): String = definedExternally
+
+    companion object {
+        val `static val`: Int = definedExternally
+        var `static var`: String = definedExternally
+
+        fun `get 🦄`(): String = definedExternally
+    }
+}
+
+fun box(): String {
+    val a = A()
+
+    assertEquals(23, a.`@invalid @ val@`)
+    assertEquals("A: before", a.`--invalid-var`)
+    assertEquals("something weird", a.`get something$weird`())
+
+    a.`--invalid-var` = "A: after"
+    assertEquals("A: after", a.`--invalid-var`)
+
+    assertEquals(42, A.Companion.`static val`)
+    assertEquals("Companion: before", A.Companion.`static var`)
+    assertEquals("\uD83E\uDD84", A.Companion.`get 🦄`())
+
+    A.`static var` = "Companion: after"
+
+    assertEquals("Companion: after", A.Companion.`static var`)
+
+    return "OK"
+}