Remove parking_lot from bevy_asset and bevy_macro_utils

[bushrat011899]

Objective

• Contributes to Tracking Issue: no_std for bevy_asset #18978 by removing a dependency that is not no_std compatible

Solution

• Removed parking_lot from bevy_asset and switched to bevy_platform::sync internally.
• Removed parking_lot from bevy_macro_utils and switched to std::sync internally. Also added a simple alternative to mapped RwLock guards while the feature is nightly-only.

Testing

• CI

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Notes

It is possible there is a performance change here, but I believe it will be negligible due to performance improvements in std since Bevy adopted parking_lot in #210. If there is a performance difference noticed, I believe the more appropriate course of action would be to move parking_lot into bevy_platform as a feature, so that all parts of Bevy could benefit, not just bevy_asset.