Add Align::Fill variant

[tarkah]

Adds a new Align::Fill variant that works similar to align-items: stretch;.

Currently, it's impossible to have all child elements of a row or column "fill" the remaining space based on the largest sized child. This makes using widgets like Rule impossible when nested inside a row or column that has no fixed size.

Here's an example of how it's possible to use Rule in the pokedex example, where it's height now stretches to fill the dynamic height of the row it's in. Without Align::Fill, the Rule expands to fill the entire viewport.

Before:

After:

Closes #746

[hecrj]

Looks great! 🥳

It's really nice that we can extend the layout engine so easily with just an additional variant! 🎉

I left a couple of suggestions. Let me know what you think!

[tarkah]

Thanks, @hecrj! Agreed, a surprisingly easy change for a big layout improvement.

One last question before we merge... I'm having a harder time reasoning about the potential impact of this change on Container, since it accepts Align in align_x and align_y, but it only has "one" child. Do we need to handle anything special in layout of Container?

[hecrj]

@tarkah I think the changes in Node::align should take care of it. We could also create a new enum exclusively for cross alignment.

[hecrj]

I have rebased and refactored the alignment types by moving them into a new alignment module.

Also, I have changed the Container API to leverage alignment::Horizontal and alignment::Vertical directly, which have no Fill variant.