Cargo features cannot be scoped to a build profile, so add a `dev` feature
that pulls in `bevy/debug` and `bevy/track_location` and make it a default
feature. This surfaces real type names and source locations in ECS panics
(e.g. B0001 query conflicts) for normal `cargo build` / `cargo run` /
`cargo test`, at the cost of a small overhead. Shipping builds opt out
with `--release --no-default-features`.
Removes the now-redundant `[dev-dependencies] bevy = { features = ["debug"] }`
override, since the default feature already covers test builds.
Adds bevy as a dev-dependency with the `debug` feature so that
`cargo test` builds with system-name diagnostics enabled, while
release/run builds remain unaffected.
