Add val and var as xtend keywords

[ebousse]

The current Xtend support does not highlights two important keywords of the language, namely val and var, which allow declaring variables.

Example:

class Hello {
   def sayHello() {
      val message = "Hello world"
      println(message)
   }
}

When processed with Pygments, we get:

This PR fix this problem by adding these two keywords to the appropriate list.

[Anteru]

Thank you, good find!