Include position of ? in nullable_type

[Wilfred]

Given the source code:

val foo: Int?

tree-sitter-kotlin produces:

source_file (0, 0) - (1, 0)
  property_declaration (0, 0) - (0, 13)
    val (0, 0) - (0, 3) "val"
    variable_declaration (0, 4) - (0, 13)
      simple_identifier (0, 4) - (0, 7) "foo"
      : (0, 7) - (0, 8) ":"
      nullable_type (0, 9) - (0, 13)
        user_type (0, 9) - (0, 12)
          type_identifier (0, 9) - (0, 12) "Int"

If the ? was included, I can diff it in difftastic, and it would help syntax highlighters too.