Expose a public constructor for Mut

[alice-i-cecile]

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

While ordinarily Mut should not be constructed directly by users (and instead should be retrieved via querying), it can be very useful when writing bevy-external tests.

What solution would you like?

Create a Mut::new(value: T, ticks: Ticks) method.

Carefully document that this is an advanced API, and explain how to correctly set the Ticks value (following the set_last_changed API).

What alternative(s) have you considered?

Users can already construct arbitrary Mut values directly, via APIs like world.get_mut(). This may involve creating a dummy world, which is wasteful, confusing and roundabout.

Additional context

Discussed on Discord in the context of testing asset code with @djeedai, @maniwani and @JoJoJet.