Add health bar example

[GitGhillie]

Objective

Show a way to create a health bar which follows a 3D object around.

Solution

• Created example
• Using camera.world_to_viewport() to position the UI node

Testing

• cargo run --example healthbar

Showcase

Notes

• The text showing the health is taking up a part of the health bar and I'm not sure how to fix it
• Created during the Rustweek 2025 unconf, thanks again all 😄 cc @inodentry

[GitGhillie]

Finally had the time to implement the feedback, thanks for the tips!

This is what it looks like currently:

There is more that could be improved based on what bevy_ui_anchor is doing, but maybe it's also a signal that it should be upstreamed. I'll leave that call to the experts