Allow tuples and single plugins in add_plugins, deprecate add_plugin (#8097)

# Objective

- Better consistency with `add_systems`.
- Deprecating `add_plugin` in favor of a more powerful `add_plugins`.
- Allow passing `Plugin` to `add_plugins`.
- Allow passing tuples to `add_plugins`.

## Solution

- `App::add_plugins` now takes an `impl Plugins` parameter.
- `App::add_plugin` is deprecated.
- `Plugins` is a new sealed trait that is only implemented for `Plugin`,
`PluginGroup` and tuples over `Plugins`.
- All examples, benchmarks and tests are changed to use `add_plugins`,
using tuples where appropriate.

---

## Changelog

### Changed

- `App::add_plugins` now accepts all types that implement `Plugins`,
which is implemented for:
  - Types that implement `Plugin`.
  - Types that implement `PluginGroup`.
  - Tuples (up to 16 elements) over types that implement `Plugins`.
- Deprecated `App::add_plugin` in favor of `App::add_plugins`.

## Migration Guide

- Replace `app.add_plugin(plugin)` calls with `app.add_plugins(plugin)`.

---------

Co-authored-by: Carter Anderson <[email protected]>

Author: geieredgar

src/lib.rs

@@ -99,10 +99,10 @@ impl Plugin for WinitPlugin {
                 ),
             );
 
-        app.add_plugin(AccessibilityPlugin);
+        app.add_plugins(AccessibilityPlugin);
 
         #[cfg(target_arch = "wasm32")]
-        app.add_plugin(CanvasParentResizePlugin);
+        app.add_plugins(CanvasParentResizePlugin);
 
         #[cfg(not(target_arch = "wasm32"))]
         let mut create_window_system_state: SystemState<(