Layer offset

[martinmolin]

Based on the refactor branch of bevy_tiled.

• Not sure if setting the ChunkBundle transform's translation is the correct way to do it, is there one for a whole layer?
• Not sure if this is the optimal way to add the translation to the transform.
• Added a new example with a tiny isometric map that has two layers with xy offsets (negative and positive). Not sure if this should be its own example though since it's extremely similar to the iso_main example, but it was a good way to test the functionality.

[dmtaub]

@martinmolin This looks fine to me -- BTW I merged the refactor branch this morning, if you wanted to rebase and open up a non-draft PR.