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
 }

core/builtins/native/kotlin/String.kt

@@ -25,5 +25,7 @@ public class String : Comparable<String>, CharSequence {
 
     public override fun charAt(index: Int): Char
 
+    public override fun subSequence(start: Int, end: Int): CharSequence
+
     public override fun compareTo(other: String): Int
 }