Improve bevy_reflect no_std support (#18060)
# Objective
- Fixes #18051
## Solution
- Switched function registry to use `bevy_platform_support::sync`
instead of `std::sync`
- Also documented that `debug_stack` (and transitively `debug`) require
`std`
- Also removed `std` from `wgpu-types` since that crate is now `no_std`
compatible
## Testing
- `cargo check -p bevy_reflect --no-default-features --features
critical-section,documentation,glam,glam/libm,smallvec,wgpu-types,functions
--target thumbv6m-none-eabi`
---
## Notes
With these changes, `bevy_reflect`'s feature support looks like this:
| Feature | `no_std` (Before) | `no_std` (After) | Notes |
| - | - | - | - |
| `default` | ❌ | ❌ | |
| `std` | ❌ | ❌ | |
| `documentation` | ✔️ | ✔️ | |
| `functions` | ❌ | ✔️ | |
| `critical-section` | ✔️ | ✔️ | |
| `bevy` | ✔️* | ✔️* | Partial due to `smol_str` |
| `debug` | ❌ | ❌ | |
| `debug_stack` | ❌ | ❌ | |
| `petgraph` | ❌ | ❌ | |
| `glam` | ✔️* | ✔️* | Requires enabling `glam/libm` |
| `smallvec` | ✔️ | ✔️ | |
| `uuid` | ✔️* | ✔️* | Requires setting up `getrandom` backend |
| `wgpu-types ` | ❌ | ✔️ | |
| `smol_str` | ✔️* | ✔️* | Partial due to `smol_str` not supporting
`portable-atomic` |
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