Add picking support for tiles

[Buncys]

Summary

This PR adds native picking support, as requested in #572.

Usage

bevy_ecs_tilemap = { version = "0.19", features = ["picking"] }

Picking events are reported on the tile entity (not the tilemap entity). Attach observers like this:

commands.entity(tile_entity).observe(|click: On<Pointer<Click>>| { /* ... */ });

See the picking example (cargo run --example picking --features picking).

Implementation

Mostly based on this comment by @TheNeikos (thanks!).

Fixes #572

[Buncys]

Looks like CI keeps failing on dead-code warnings in examples/helpers/ldtk.rs, unrelated to this PR. Reproducible in main.