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

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

@@ -0,0 +1,47 @@
+// IGNORE_BACKEND: JS
+// !LANGUAGE: +JsAllowInvalidCharsIdentifiersEscaping
+
+package foo
+
+@JsExport()
+class A {
+    val `#invalid@char value`: Int = 41
+    val __invalid_char_value: Int = 23
+
+    var `--invalud char@var`: String = "A: before"
+}
+
+class B {
+    val `#invalid@char value`: Int = 42
+    val __invalid_char_value: Int = 24
+
+    var `--invalud char@var`: String = "B: before"
+}
+
+fun box(): String {
+    val a = A()
+    val b = B()
+
+    assertEquals(23, a.__invalid_char_value)
+    assertEquals(24, b.__invalid_char_value)
+
+    assertEquals(41, a.`#invalid@char value`)
+    assertEquals(42, b.`#invalid@char value`)
+
+    assertEquals("A: before", a.`--invalud char@var`)
+    assertEquals("B: before", b.`--invalud char@var`)
+
+    a.`--invalud char@var` = "A: after"
+    b.`--invalud char@var` = "B: after"
+
+    assertEquals("A: after", a.`--invalud char@var`)
+    assertEquals("B: after", b.`--invalud char@var`)
+
+    assertEquals(41, js("a['#invalid@char value']"))
+    assertEquals(js("undefined"), js("b['#invalid@char value']"))
+
+    assertEquals("A: after", js("a['--invalud char@var']"))
+    assertEquals(js("undefined"), js("b['--invalud char@var']"))
+
+    return "OK"
+}