Prohibit illegal identifiers in packages in JS BE

See KT-18032

compiler/testData/diagnostics/testsWithJsStdLib/name/illegalPackageName.kt

@@ -0,0 +1,3 @@
+package <!INVALID_CHARACTERS!>`//`<!>
+
+class A

compiler/testData/diagnostics/testsWithJsStdLib/name/illegalPackageName.txt

@@ -0,0 +1,11 @@
+package
+
+package `//` {
+
+    public final class A {
+        public constructor A()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}