Makes use of `std` explicit, simplifying a possible `no_std` port.
# Objective
- Contributes to #15460
- Simplify future `no_std` work on `bevy_asset`
## Solution
- Add `#![no_std]` to switch to `core::prelude` instead of
`std::prelude`
## Testing
- CI
---
## Notes
This is entirely a change around the names of imports and has no impact
on functionality. This just reduces the quantity of changes involved in
the (likely more controversial) `no_std`-ification of `bevy_asset`.
8f32c799ee