Add `Has<C>` `WorldQuery` type to replace `Option<&C>` when checking existence

[alice-i-cecile]

What problem does this solve or what need does it fill?

I commonly find myself reaching for Option<&C> when I just need to check whether or not a component exists, and then branching on Some versus None.

This works well enough, but:

1. The access is more restrictive than it needs to be: we request read access, but only actually need existence checking. This limits parallelism, as these systems could run in parallel with mutating systems.
2. It's less clear to read: the pattern is a bit strange and implicit.
3. It may be measurably slower.

What solution would you like?

Add a Has<C: Component> type. Implement WorldQuery for this, with no access required.

The item type of each query element should be bool.

What alternative(s) have you considered?

We could do nothing, the eixisting pattern isn't terrible.

We could name this Is<C>: I'm not sure which one is clearer.

[SkiFire13]

It's pretty horrible but Or<(With<T>,)> (when used as query rather than filter) does yield a bool indicating whether the T is present or not. This is kinda weird though since With<T> by itself does not do this.

[alice-i-cecile]

Ah right, that's caused by #7680.

[BoxyUwU]

Option<With<T>> works which is nicer than Or<(With<T>,) imo

[BoxyUwU]

sidenote: Or does not return bool it returns ()