Add isEmpty, isBlank methods and their nullable and negating couterparts.

js/js.libraries/src/core/stringsCode.kt

@@ -28,6 +28,9 @@ public inline fun String.matches(regex : String) : Boolean {
     return result != null && result.size() > 0
 }
 
+
+public fun String.isBlank(): Boolean = length() == 0 || matches("^[\\s\\xA0]+$")
+
 public fun String.equals(anotherString: String, ignoreCase: Boolean = false): Boolean =
         if (!ignoreCase)
             this == anotherString