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

core/builtins/native/kotlin/CharSequence.kt

@@ -20,4 +20,6 @@ public trait CharSequence {
     public fun length(): Int
 
     public fun charAt(index: Int): Char
+
+    public fun subSequence(start: Int, end: Int): CharSequence
 }