Checks whether the the field is allowed to automatically decide its own height.
This directly corresponds to the editor setting for the height of the extended field. If it is set to "auto", this will return true. If it's set to a number of rows for height, then this will return false.
This works in conjunction with FieldAdapter.autosizeBehavior(). This method allows the extension to ask if the editor definitions support autosizing. FieldAdapter.autosizeBehavior() is Agentry's way of asking the extension how to handle autosizing.