Add <Stack> (a variant of <Flex>)

[ravicious]

This PR adds a Flex variant called Stack. It's basically just <Flex flexDirection="column" alignItems="flex-start">. Over time, I found myself reaching for it pretty often, especially in the new VNet diagnostic report that I'm working on. It gets pretty annoying to define it by hand whenever you want to use it, hence this PR.

It's based on The Stack from Every Layout:

Flow elements require space (sometimes referred to as white space) to physically and conceptually separate them from the elements that come before and after them. This is the purpose of the margin property.

However, design systems conceive elements and components in isolation. At the time of conception, it is not settled whether there will be surrounding content or what the nature of that content will be. One element or component is likely to appear in different contexts, and the requirement for spacing will differ.

We are in the habit of styling elements, or classes of elements, directly: we make style declarations belong to elements. Typically, this does not produce any issues, but margin is really a property of the relationship between two proximate elements. (…)

The trick is to style the context, not the individual element(s). The Stack layout primitive injects margin between elements via their common parent.

I've used gap instead of margin-block like Every Layout does because 1) it's what we already use 2) it has wider browser support.

Basic example	Example with margin-top: auto

[avatus]

YESSSS

[public-teleport-github-review-bot]

@ravicious See the table below for backport results.

Branch	Result
branch/v17	Create PR

[public-teleport-github-review-bot]

@ravicious See the table below for backport results.

Branch	Result
branch/v16	Failed
branch/v17	Failed