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

compiler/testData/codegen/box/strings/kt5956.kt

@@ -0,0 +1,15 @@
+// KT-5956 java.lang.AbstractMethodError: test.Thing.subSequence(II)Ljava/lang/CharSequence
+
+class Thing(val delegate: CharSequence) : CharSequence {
+    override fun charAt(index: Int): Char {
+        throw UnsupportedOperationException()
+    }
+    override fun length(): Int = 0
+    override fun subSequence(start: Int, end: Int) = delegate.subSequence(start, end)
+}
+
+fun box(): String {
+    val txt = Thing("hello there")
+    val s = txt.subSequence(0, 1)
+    return if ("$s" == "h") "OK" else "Fail: $s"
+}