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.translator/testData/expression/string/cases/subSequence.kt

@@ -0,0 +1,15 @@
+package foo
+
+fun box(): String {
+    val kotlin: String = "kotlin"
+
+    if (kotlin.subSequence(0, kotlin.length()) != kotlin) return "Fail 0"
+
+    val kot: CharSequence = kotlin.subSequence(0, 3)
+    if (kot.toString() != "kot") return "Fail 1: $kot"
+
+    val tlin = (kotlin : CharSequence).subSequence(2, 6)
+    if (tlin.toString() != "tlin") return "Fail 2: $tlin"
+
+    return "OK"
+}