Add associated constant `IDENTITY` to `Transform` and friends.

[irate-devil]

Objective

Since identity is a const fn that takes no arguments it seems logical to make it an associated constant. This is also more in line with types from glam (eg. Quat::IDENTITY).

Migration Guide

The method identity() on Transform, GlobalTransform and TransformBundle has been deprecated. Use the associated constant IDENTITY instead.

[mockersf]

while marking as deprecated is nice, at this stage of Bevy I would prefer to break directly

[mockersf]

bors r+

[bors[bot]]

Pull request successfully merged into main.

Build succeeded:

• build-and-install-on-iOS
• build-android
• build (macos-latest)
• build (ubuntu-latest)
• build-wasm
• build (windows-latest)
• check-compiles
• check-doc
• check-missing-examples-in-docs
• ci
• markdownlint
• run-examples
• run-examples-on-wasm
• run-examples-on-windows-dx12