Update to Bevy 0.15

[ptsd]

• Updates bevy to 0.15
• Slightly tidies up the 2d example
• Adds the 2d example to Cargo.toml

[ptsd]

Seems there are two almost identical PRs 😄

[Shatur]

Merged #15 since I planning to remove example mentions from Cargo.toml and similar comments from the 3D example 😅