tileset looks_like: further enhancements

[mlangsdorf]

the implementation of looks_like in #24747 is adequate, but people have been asking for further improvements. I would like to get the basic implementation into the player base as quickly as possible, so I'm not going to make those changes in 24747. But they're good suggestions, so I'm going to track them here.

• [ ] auto-assign looks_like for artifacts: add a randomly(?) assigned looks_like to artifacts.
• [x] handle overlays: overlays use a different code path than most tileset items. It's not impossible to add support for them, but I missed the code path and there's some string handling issues that will make it tricky. See #25656
• [ ] expand looks_like: looks_look should default to a string, matching the ID of another object in the same category, but should have the option of being written as an object or list of the form { "object": "CATEGORY" } or [ "object", "CATEGORY" ] - probably the first. Then looks_like could use tile images from other categories, which might be useful for terrain and furniture at least. Bonus would be removing a lot of the redundant code and boy I wish I'd thought of this yesterday.

Assign to me, please.

[mlangsdorf]

Artifacts are items. They'll be able to get a looks_like field. but all a looks_like field does is tell the tileset loader that if a tileset doesn't have a tile for an object, it should use the tile for the looks_like object, if any. It doesn't create dummy objects or require tilesets to have a tile for an object or anything like that. It if very simple if a tileset doesn't have tile A, but does have tile B, use B for A if A looks_like B. Looking at the artifact code, it looks like it would pretty easy to give most artifact shapes a looks_like field that you could reasonably expect more tilesets to have. 5 weapons, 5 tools, 7 armors, 16 natural shapes. But even if a scaled_sphere has "looks_like": "grenade", if the tileset doesn't have a grenade or a looks_like chain from grenade that resolves into a tile, you're still going to see an ASCII symbol.

[mlangsdorf]

Looked into doing this, and I was wrong: artifacts aren't items. They're like items, but subtly different.

The key problem in this case is that it doesn't look like artifacts get passed to the tileset loader at all, or at least not in a recognizable form. It's easy, if tedious, for me to add a bunch of "looks_like" to the relevant artifact generation tables. It will take more work to add support for them in the tileset loader.