defer Closure evaluation for append() and prepend()

[gp-lnuff]

Currently, the prepend() and append() methods evaluate a Closure passed to them as soon as it is given.
This creates issues when we want to inject utilities such as $livewire in the Closure - it will throw an Exception along the lines of "$container must not be accessed before initialization".

With this PR, two new methods are added:

• getAppendedItems(), which evaluates $this->append and performs the checks on the value that were moved from append()
• getPrependedItems(), which evaluates $this->prepend and performs the checks on the value that were moved from prepend()

Then, in getTree(), we wrap $this->append and $this->prepend in a filled() call, so that the when() does not evaluate the Closure prematurely, and we use the two new methods to retrieve the appended/prepended values, with checks, only if they need to be added to the tree.

I have another change lined up that depends on this deferral of the Closure evaluation to handle multiple appended/prepended values, but I chose to separate the two since this is more of a bugfix while the other is an enhancement.

[CodeWithDennis]

Thanks!