Allow queries to work without mutable world access

[ElliottjPierce]

Objective

Bevy is considering not allowing mutable world access in WorldQuery::init_state.
See here.
This PR explores how this crate might migrate if those changes go through.

Solution

Don't initialize the registry resource in init_state.
Use atomics to seal the registry.

There is one caveat.
Before this pr, if a component was registered for a trait after the query was made, it would panic.
After this pr, that panic will only happen if, when the query was made, there were already components registered for that trait.

Situation	previous behavior	new behavior
All components registered before first query	Works as expected	Works as expected
Some components registered before first query and some after	Panics on late registration	Panics on late registration
No components are registered	Warns	Warns
No components are registered before a query is made, but then some are registered	Warns and panics	Just warns

Since both the warning and panic serve the same purpose (to remind users to actually register their components), I don't think this is a concern, but I wanted to be transparent about it.