Bevy TimeUpdateStrategy cannot move time backwards

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

Currently it is possible to use [ManualInstant](https://docs.rs/bevy/latest/bevy/time/enum.TimeUpdateStrategy.html#variant.ManualInstant) to set the `Time` to a specific `Instant`.

This can be used to e.g. track which frame you are simulating and step single frames while choosing how much time elapses in-between frames.

However, setting an earlier `Instant` results in no delta (and no change in elapsed) since the [Bevy time implementation](https://github.com/bevyengine/bevy/blob/main/crates/bevy_time/src/real.rs#L93) does subtraction on `Instant`s which saturates to a zero duration if a larger instant is subtracted from a lesser instant.

This prevents uses cases like

- Simulation frame stepping backwards
- Creating a "scrubber" like in video editing software which is able to move backwards

## What solution would you like?

Make setting earlier instants in `ManualInstant` affect the elapsed time.