Add World::get_resource_or_init as an alternative to World::get_resource_or_insert_with

[ItsDoot]

Objective

If a Resource implements FromWorld or Default, it's nicer to be able to write:

let foo = world.get_resource_or_init::<Foo>();

Rather than:

let foo = world.get_resource_or_insert_with(Foo::default);

The latter is also not possible if a type implements FromWorld only, and not Default.

Solution

Added:

impl World {
    pub fn get_resource_or_init<R: Resource + FromWorld>(&mut self) -> Mut<'_, R>;
}

Turns out all current in-engine uses of get_resource_or_insert_with are exactly the above, so they've also been replaced.

Testing

• Added a doc-test.
• Also added a doc-test for World::get_resource_or_insert_with.