Add marker components for different camera types

[alice-i-cecile]

What problem does this solve or what need does it fill?

Selecting particular camera types is frustrating, relying on checking the name field within a system.

What solution would you like?

Add a simple marker components for UiCamera and GameplayCamera.

Various rendering systems should use this mechanism, instead of the camera's name.

What alternative(s) have you considered?

Users can add their own marker components manually.

Only tag UiCamera and use Without<UiCamera>.

Add a unique marker component for each of 2dCamera, 3dCamera and so on. This is less clear, and it seems extremely rare that you will have more than one different type of gameplay camera at once.

Additional context

Example of this problem in #help

[alice-i-cecile]

As part of the PR to fix this issue, it might be nice to demonstrate how to use it in a small example (or as part of one of the games). Combine it with query.single_mut() to show idiomatic usage.

[alice-i-cecile]

While we're fixing this, we should probably just rework how Bevy's internal systems manage cameras as well.

It's probably worth removing the name field entirely, to avoid exposing / using an inferior API. Now that sparse set storage exists, there's no reason not to use marker components for all of the "name X" use cases.

[alice-i-cecile]

This resulted in a confusing problem for a user trying to make their own camera variant.

[alice-i-cecile]

#3412 will significantly impact how this is handled and add complexity, so this should wait until after it's merged.

[james7132]

Is this still relevant after #4745? UiCamera and it's bundle are no longer a thing.