Simplify bevy_reflect Features

[bushrat011899]

Objective

Now that bevy_platform::cfg is merged, we can start tidying up features. This PR starts with bevy_reflect.

Solution

• Removed critical-section feature (it was just a re-export of bevy_platform anyway)
• Removed std and web features, relying on bevy_platform::cfg to check for availability.
• Made smol_str implicit feature explicit.
• Refactored various parts of bevy_reflect to use the new cfg syntax where appropriate.

Testing

• CI

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Notes

• Certain attribute-style feature gates can't easily be replaced with the newer macro_rules! style (e.g., enum variants, struct fields). This is blocked on rust/#54727. See macro_rules_attribute for the current state-of-the-art in stable Rust.
• This PR isn't dependent on any others, but bevy_reflect: Split up the std module #18939 should be merged first and I will resolve the merge conflicts.

[MrGVSV]

Looks great!!

I'll be honest, though, I did only skim the std changes 😅. I think you're right that it will be a lot easier to review once #18939 is merged