Competing naming conventions for types

Some traits have competing naming conventions for types within Bevy:

| Trait | Suffixes used | Suggestion |
|-|-|-|
| `SystemSet` | `System`, `Set`, `Systems`, no-suffix | Not obvious which to choose, probably `Systems` |
| `Primitive3d` | `3d`, `3`, `3A`, no-suffix | Probably fine as-is |
| `Primitive2d` | `2d`, `2`, no-suffix | Probably fine as-is |
| `Event` | `Event`, no-suffix | Probably fine as-is |
| `ScheduleLabel` | `Schedule`, no-suffix | Just rename `ExtractSchedule` |
| `RenderLabel` | `Label`, no-suffix | Just rename `CameraDriverLabel` |
| `SystemParam` | `Param`, no-suffix | Just rename `UiLayoutSystemRemovedComponentParam` |
| `PluginGroup` | `Plugins` | :white_check_mark: (not counting `PluginGroupBuilder`) |
| `Plugin` | `Plugin` | :white_check_mark: |
| `Bundle` | `Bundle` | :white_check_mark: |
| `AppLabel` | `App` | :white_check_mark: |
| `System` | `System` | :white_check_mark: |
| `Material` | `Material` | :white_check_mark: |
| `Component` | no-suffix | :white_check_mark: |
| `Resource` | no-suffix | :white_check_mark: |
| `Command` | no-suffix | :white_check_mark: |
| `Asset` | no-suffix | :white_check_mark: |
| `States` | N/A | :white_check_mark: (no types provided by Bevy) |
| `EntityCommand` | N/A | :white_check_mark: (no types provided by Bevy) |

Hard to search for because no distinguishing trait:

- Observable events
- Marker components

Start by searching the listed traits and example types throughout the Bevy repository to inventory their current names and usages. Done means agreeing on a consistent naming convention, resolving the open cases in the table, and addressing the searchability concerns for observable events and marker components.

Written by the indexing model from the issue text.