New alignment / justification icons

[jameskoster]

Work under way in #47134 will add some new alignment and justification options for blocks, specifically align-items: stretch and justify-content: space-between equivalents.

We don't currently have dedicated icons for this purpose, and so should expand the set. The problem is that the design convention we've used for alignment icons so far makes use of a single item to represent the object being aligned, and it is quite tricky to represent space-between (and stretching to a lesser extent) with a single item:

Let's explore some alternative conventions.

[jameskoster]

Here's an option that uses two items instead of one, which makes it much easier to communicate space-between and stretch. There's a version with 'guide' lines and one without:

cc @WordPress/gutenberg-design

These are in the WordPress Design Library Figma file in case anyone would like to riff on them.

[jameskoster]

Option 4: Fewer guide lines (I think we probably need to keep them for stretch and space-between):

[jasmussen]

These are getting closer. They still seem a bit big, and it might be good to see them in context of the block toolbar 🤔

[jasmussen]

I think these work well alongside the existing ones:

Can we try these with 1 cell between the lines, instead of 2?

[jameskoster]

One cell can probably work in the mean time:

[jasmussen]

Yep, that seems a good interim.