Add CharSequence.subSequence

This is done primarily for JVM interoperability, otherwise it's impossible to
inherit from CharSequence there. On JS subSequence at the moment just invokes
substring.

 #KT-5956 Fixed

js/js.tests/test/org/jetbrains/k2js/test/semantics/StringTest.java

@@ -90,4 +90,8 @@ public final class StringTest extends AbstractExpressionTest {
     public void testNullableTypeInStringTemplate() throws Exception {
         checkFooBoxIsOk();
     }
+
+    public void testSubSequence() throws Exception {
+        checkFooBoxIsOk();
+    }
 }