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/js/src/kotlin/string.kt

@@ -24,10 +24,19 @@ public actual fun String(chars: CharArray, offset: Int, length: Int): String {
     return String(chars.copyOfRange(offset, offset + length))
 }
 
-
+/**
+ * Returns a copy of this string converted to upper case using the rules of the default locale.
+ *
+ * @sample samples.text.Strings.toUpperCase
+ */
 @kotlin.internal.InlineOnly
 public actual inline fun String.toUpperCase(): String = asDynamic().toUpperCase()
 
+/**
+ * Returns a copy of this string converted to lower case using the rules of the default locale.
+ *
+ * @sample samples.text.Strings.toLowerCase
+ */
 @kotlin.internal.InlineOnly
 public actual inline fun String.toLowerCase(): String = asDynamic().toLowerCase()