Add support for object templates

[Cleptomania]

Object templates are separate files which store a template that can be applied to an object which specifies itself as an instance of that template.

See: https://doc.mapeditor.org/en/stable/reference/json-map-format/#object-template

[Cleptomania]

This has been done.

Initial implementation in fec4064

Initial tests in e2573ff

As far as test coverage goes, the coverage report is good, however due the nature of templates it can't really know all the edge cases. Would be nice to have a test which covers templates of every type of object at least, if not several of each. But ya know, time and effort and all that.