Link underline variant

[microbit-grace]

https://microbit-foundation.github.io/ui/?path=/story/typography-link--inline-in-text

Links currently rely on text colour to show they are links. Though in some cases it is obvious they are links by placement or the text, if a text is inlined it is not as obvious. It is also not very accessible for people who can't spot the colour difference.

Perhaps we could have an underline variant, similar to Chakra V3 (https://chakra-ui.com/docs/components/link#variants)?

[microbit-matt-hillsdon]

Yeah this is definitely a gap and one I want to come back to post-conversion as we probably need to make some visible change too (e.g. Python Editor has non-underlined links in prose).