Update to bevy 0.15

[benfrankel]

Fixes #320.

We'll probably want to set up a tag / versioning system soon.

The ChildBuild workaround I chose is probably not ideal, but it works.

Confirmed that the game compiles, runs, and seems fine.

[bas-ie]

Also checked and working here! No idea what to say about the ChildBuild issue, but guess it doesn't block a merge?

[bas-ie]

BTW I think the CI failure here is caused by dynamic_linking no longer working with cargo test --doc, have just been encountering this in another third-party crate.

[alec-deason]

I think this error can be fixed by adding export LD_LIBRARY_PATH=`rustc --print target-libdir` or the equivalent to be CI job before cargo test runs.

[benfrankel]

I can reproduce the CI failure on my own machine by running cargo test --doc. Running the same command on main also fails, so the failure seems to be unrelated to this PR.

This can also be seen with the same CI failure in #322, which only touches README.md.

EDIT: Upon further investigation, I see that the CI doctest failures started when the rustc toolchain changed from version 1.82.0 to 1.83.0. I also confirmed locally that either disabling the bevy/dynamic_linking feature OR prepending LD_LIBRARY_PATH="$(rustc --print target-libdir)" to the cargo test --doc command fixes the error.

This would be a silly reason to disable bevy/dynamic_linking, so I'm opting for the latter fix.