Create samples for String.toLowerCase/toUpperCase

* Create toLowerCase method sample
* Create toUpperCase method sample
* Create toUpperCase sample reference in StringsJVM
* Create sample ref and doc of js impl toUpperCase
* Create sample ref and docs of js toLowerCase
* Add sample ref in toLowerCase expect fun
* Add sample ref in toUpperCase expect fun

libraries/stdlib/common/src/kotlin/TextH.kt

@@ -111,8 +111,18 @@ internal expect fun String.nativeLastIndexOf(str: String, fromIndex: Int): Int
 public expect fun String.substring(startIndex: Int): String
 public expect fun String.substring(startIndex: Int, endIndex: Int): String
 
-
+/**
+ * Returns a copy of this string converted to upper case using the rules of the default locale.
+ *
+ * @sample samples.text.Strings.toUpperCase
+ */
 public expect fun String.toUpperCase(): String
+
+/**
+ * Returns a copy of this string converted to lower case using the rules of the default locale.
+ *
+ * @sample samples.text.Strings.toLowerCase
+ */
 public expect fun String.toLowerCase(): String
 public expect fun String.capitalize(): String
 public expect fun String.decapitalize(): String