Add `world` and `world_mut` methods to `WorldChildBuilder`

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

Allow `World` access when you only have a `WorldChildBuilder`.

## What solution would you like?

Add `world` and `world_mut` methods to `WorldChildBuilder`, returning references to the inner `&mut World`. This doesn't have to be `unsafe` like `EntityWorldMut::world_mut`, because the latter is only unsafe due to the `EntityLocation` stored in `EntityWorldMut` that could become invalidated. There is no `EntityLocation` stored in `WorldChildBuilder`.

## What alternative(s) have you considered?

None.

Locate the definition and construction path of WorldChildBuilder, then read the related EntityWorldMut world access methods for comparison. Done means WorldChildBuilder exposes world and world_mut methods returning references to its inner World without requiring unsafe access, with coverage added wherever this API's existing tests live.

Written by the indexing model from the issue text.