export `resource_entity` in `EntityWorldMut` and `TemplateContext` (#24292)

# Objective

- Provide a way for accessing resource entities, enabling the use of
#19731 .

## Solution

- Add `resource_entity::<R: Resource>` using `World::resource_entities`.
- Not sure if there is needed for accessing `ResourceEntities`. Just
exported `resource_entities` in `EntityWorldMut` but not in
`TemplateContext`.

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>