Add screen/world point transformation helpers

[dantheman3333]

Bevy should include some helpers similar to Unity's Camera.ScreenToWorldPoint and Camera.WorldToScreenPoint

[GabCampbell]

Hey Thanks for the suggestion? Do you have a suggestion on how it could be integrated to bevy?

[smokku]

There is a raycasting code in https://github.com/aevyrie/bevy_mod_picking - maybe it could be exposed for stand-alone use.

[dantheman3333]

Hey Thanks for the suggestion? Do you have a suggestion on how it could be integrated to bevy?

No problem. I started to look into it last week but I've been really busy with work/life, so I wanted to open it up to discussion now in case someone as already implemented it in their games or have experience with it.

[aevyrie]

See #1258 for world_to_screen.
#1799 would need to be extended with a function that accepts z-depth to return a Vec3 in world space instead of a line.

[tim-blackbird]

I think this can be closed since #6126 landed.