Update PanCam and FreeCam to use full term Camera

[syszery]

Objective

This PR addresses #21569, which proposes renaming the newly introduced camera controller modules FreeCam and PanCam to use the full term Camera.

This PR resolves the PanCam portion.

Solution

• Renamed the PanCam controller, PanCamPlugin, and related methods to use the full term Camera instead of the abbreviation Cam.
• Renamed the module from pan_cam to pan_camera for consistency with naming conventions.
• Updated the example pan_camera_controller and adjusted usage of the renamed controller and plugin.
• Updated documentation and release notes accordingly.

Follow-up Work

I see two options from here:

1. Use this PR as a reference for renaming FreeCam. The process is similar and could be a great first issue for someone looking to contribute to the new camera modules or Bevy in general. Most of the changes follow the same pattern, although FreeCam has more examples that need updating. One could find them using grep (e.g., grep FreeCam) or by reviewing the diff from the PR that introduced FreeCam: Add bevy_camera_controllers crate and move freecam implementation into it #20215

2. I can continue and update this PR to also handle the FreeCam renaming, if you'd prefer to resolve the entire issue in one go.

[alice-i-cecile]

I can continue and update this PR to also handle the FreeCam renaming, if you'd prefer to resolve the entire issue in one go.

Please do this :)

[syszery]

I've updated both available camera controllers to consistently use the full term "camera", replacing previous uses of "cam". I aimed to be consistent across crate modules and examples while keeping doc changes minimal and non-invasive.

There’s one remaining detail in free_camera_controller.rs I’d like feedback on:

In the following snippet, I’d like to rename the variables state and controller to camera_state and camera_controller respectively, to better reflect their roles and improve clarity:

let Ok((mut transform, mut state, controller)) = query.single_mut() else {
    return;
};

Do you agree with this change, or would you prefer different names?

Also, this now fixes #21569