Add box_align and justify for boxes

[shawna-p]

This PR adds two properties to boxes, addressing #5094 and #3666.

In particular, it has the following functionality:

justify

• True - All lines but the final line will have their items evenly distributed such that they reach the edges of the box.
• "first" - Only the first line will be evenly distributed.
• "all" - All lines, including the final line, will be evenly distributed
• False - No distribution; the default
  Lines which only have one item do not get justified (as there is no way to do so) and are considered unjustified for the purposes of box_align.

box_align

• A value (typically between 0.0 and 1.0 but not required) which will be used to align the contents of box lines which are not justified. Has no effect if box_wrap is False. Only aligns lines relative to the tallest/widest row but does not cause size changes for the box.
• It has an accompanying config value, box_reverse_align, to revert a feature that can happen with the box_reverse change introduced in 8.2.
  • If you use box_reverse True in 8.2, wrapped rows in an hbox will appear right-aligned (in the way they appeared left-aligned when box_reverse False). This would reverse the box_align property, with 0.0 aligning items to the right and 1.0 to the left, which seems unintuitive, so when box_reverse is True for an 8.2 box, I flip the value of box_align (so 0.0 becomes 1.0, 0.2 becomes 0.8, etc.).
  • Setting config.box_reverse_align = False reverts this behavior so box_align 0.0 with box_reverse True will result in right-aligned rows (and equivalents for vbox).

NOTE

I would like box_align to be a proper style property, alongside other box_ and alignment properties, and have structured my PR such that it is expected to be a style property. However, I believe there is some piece I am missing as it is not recognized as a style property in my tests. Therefore, this code does not work directly as it throws an error about box_align not being a recognized style property.

I have, however, tested this code by making it a keyword instead and it is functioning as expected. If needed I can adapt the PR to make it a keyword instead.

Merging this PR should also be followed up by including a line in the Incompatible section explaining config.box_reverse_align for 8.2 box_reverse compatibility.