Allow a closure to be used as a required component default

[fluffiac]

Objective

Allow required component default values to be provided in-line.

#[derive(Component)]
#[require(
    FocusPolicy(block_focus_policy)
)]
struct SomeComponent;

fn block_focus_policy() -> FocusPolicy {
    FocusPolicy::Block
}

May now be expressed as:

#[derive(Component)]
#[require(
    FocusPolicy(|| FocusPolicy::Block)
)]
struct SomeComponent;

Solution

Modified the #[require] proc macro to accept a closure.

Testing

Tested using my branch as a dependency, and switching between the inline closure syntax and function syntax for a bunch of different components.

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[alice-i-cecile]

I quite like this, but this needs user facing docs demonstrating that this is possible.