Add App::setup_non_send, deprecate insert_non_send

[Supremesv715]

Objective

• App::insert_non_send constructs !Send values immediately at the app-building call site, which can put them on the wrong thread relative to the runner (e.g. WinitPlugin).
• Provide a deferred API so !Send data is created on the thread that actually runs the app.

Solution

• Add App::setup_non_send, which queues a Send closure and runs it once at the start of App::run on the calling thread.
• Deprecate App::insert_non_send (and update the insert_non_send_resource note) in favor of setup_non_send.
• Migrate examples/app/log_layers_ecs.rs and add a migration guide.
• App::init_non_send is unchanged.

Testing

• Added unit tests covering deferred execution and FIFO ordering of queued setups.
• Reviewers can run:
  • cargo test -p bevy_app setup_non_send
  • cargo run --example log_layers_ecs

[eswartz]

Makes sense to me!

[eswartz]

A nit: it seems like the primary explanation comment shouldn't justify itself against a deprecated method, but instead just make its own case as the following sentences mostly do. (Not to mention there's no "func" to "run immediately" in insert_non_send anyway :)

IMHO the PR description and migration guide explain why this method exists; once the deprecated methods are removed, the comments submitted here should stand alone without editing.