Provide String.commonPrefixWith and String.commonSuffixWith methods.

Char.isHighSurrogate and Char.isLowSurrogate now available in js-stdlib too.

libraries/stdlib/test/text/StringTest.kt

@@ -70,6 +70,39 @@ class StringTest {
         assertFalse("".endsWith('a'))
     }
 
+    test fun commonPrefix() {
+        assertEquals("", "".commonPrefixWith(""))
+        assertEquals("", "any".commonPrefixWith(""))
+        assertEquals("", "".commonPrefixWith("any"))
+        assertEquals("", "some".commonPrefixWith("any"))
+
+        assertEquals("an", "annual".commonPrefixWith("any"))
+        assertEquals("an", "annual".commonPrefixWith("Any", ignoreCase = true))
+        assertEquals("", "annual".commonPrefixWith("Any", ignoreCase = false))
+        // surrogate pairs
+        val dth54 = "\uD83C\uDC58" // domino tile horizontal 5-4
+        val dth55 = "\uD83C\uDC59" // domino tile horizontal 5-5
+        assertEquals("", dth54.commonPrefixWith(dth55))
+        assertEquals(dth54, "$dth54$dth54".commonPrefixWith("$dth54$dth55"))
+
+    }
+
+    test fun commonSuffix() {
+        assertEquals("", "".commonSuffixWith(""))
+        assertEquals("", "any".commonSuffixWith(""))
+        assertEquals("", "".commonSuffixWith("any"))
+        assertEquals("", "some".commonSuffixWith("any"))
+
+        assertEquals("ly", "yearly".commonSuffixWith("monthly"))
+        assertEquals("strö", "strö".commonSuffixWith("BISTRÖ", ignoreCase = true))
+        assertEquals("", "yearly".commonSuffixWith("HARDLY", ignoreCase = false))
+        // surrogate pairs
+        val dth54  = "\uD83C\uDC58" // domino tile horizontal 5-4
+        val kimono = "\uD83D\uDC58" // kimono
+        assertEquals("", dth54.commonSuffixWith(kimono))
+        assertEquals("$dth54", "d$dth54".commonSuffixWith("s$dth54"))
+    }
+
     test fun capitalize() {
         assertEquals("A", "A".capitalize())
         assertEquals("A", "a".capitalize())