Fix focus style being overlapped by summary action links

[querkmachine]

Fixes the WCAG 2.2 issue of subsequent action links in the summary list component slightly overlapping the currently focused link.

Resolves #3841.

Screenshots

	Before	After
Narrow layout
Wide layout

Changes

• Summary list action links now receive the isolation: isolate style when focused. This creates a new stacking context, making it appear 'above' the surrounding content.

Thoughts

• There are a few ways that creating a new stacking context could have been done. position: relative, transform: translate(0, 0), and others. I've used the isolation property as it is intended specifically for this purpose, on the assumption this fix doesn't need to support IE. (Browser support for isolation)
• This issue doesn't affect the action links on summary cards, as they are already separated using row-gap, preventing the overlap from occuring.
  • Presumably we don't want to do the same thing here, as the action link line spacing is consistent with the content next to it and we probably don't want to change that.

[colinrotherham]

This is great, hadn't seen isolation before

Could it have been solved with "target size" instead as those link heights are quite low on mobile?

• Consider larger elements for a bigger touch/target size #2060

[querkmachine]

@colinrotherham My presumption is that the current spacing is intentional, as it means the links have a vertical rhythm consistent with the text next to it.

If we increase the size or spacing of the links, we lose that bit of visual consistency.

As there aren't usually more than a 2 or 3 action links (this being a contrived, extreme example), the target size issue probably doesn't manifest itself in a problematic way very often.

The links are also (just barely) large enough to not fall foul of the WCAG 2.2 minimum target size in their current form.

[colinrotherham]

Let's approve because it fixes what it claims to fix 😎

Regarding link heights (blocks of text are exempt?) there's a similar comment regarding Check your answers "Change" links but it's one for another issue

[dav-idc]

Cleanup Q: should this issue be in the govuk-frontend 5.0.0 milestone?

[dav-idc]

Ah never mind, I see the issue associated with this pull request is in the 5.0.0 milestone 👍

Here's a full list of the accessibility issues addressed in 5.0, from what I can tell: https://github.com/alphagov/govuk-frontend/issues?q=milestone%3Av5.0+label%3Aaccessibility