Improve bevy_reflect no_std support

[bushrat011899]

Objective

• Fixes bevy_reflect: no no_std support when the functions feature is enabled #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

[shekohex]

It works! Thank you for the quick fix!