Make BEVY_REFLECT_AUTO_REGISTER_STATIC require a target dir

[eugineerd]

Objective

Fix #21444

Solution

Require setting explicit target dir in BEVY_REFLECT_AUTO_REGISTER_STATIC env variable.

Right now there is no stable way to obtain this dir in a proc macro, and since we already have to set an env var for auto_register_static approach to work might as well reuse it here. BEVY_REFLECT_AUTO_REGISTER_STATIC must be an absolute path and point to the target dir for the current workspace, otherwise cargo would put type registration functions in some internal dir for each crate. This dir can be set manually or by running cargo metadata and getting target_directory field from the output json, which I've added to the example instructions.

Testing

• Tested using the original repro (https://github.com/occuros/bevy_0_17_type_registry)
• Only tested on linux

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Please review the instructions for writing migration guides, then expand or revise the content in the migration guides directory to reflect your changes.

[alice-i-cecile]

This doesn't look like it's strictly breaking, but a migration guide to help users (and search engines) figure out what's going wrong feels like it would be helpful here.

Fix itself looks good, and the docs are nice and clear.