Make reflection optional

[atlv24]

Objective

• Make it possible to disable reflection

Solution

• add a default "enable_codegen" feature which must be set, otherwise all bevy_reflect proc macros behave as no-op passthrough.
• forward it to root cargo toml through bevy_internal, add it to default app feature collection
• make all existing usage of bevy_reflect in the repo default-features=false so that this feature collection is the only thing enabling it

Testing

• this is entirely untested, i didnt even check if it compiled im going to sleep

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-22721

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.