Upgrade unified to latest v10.x

[watson]

We're in the process of upgrading the unified package in Kibana (PR: elastic/kibana#151276). However to not run two different versions of unified in Kibana we prefer if we could upgrade this package in EUI first (source). Then we'll wait with our upgrade until a new version of EUI with the upgraded package is released.

[cee-chen]

@watson is this something you're asking the EUI team to do, or is this a contribution you could help make to the EUI repo? My concern here is that I've previously investigated generally upgrading remark-related plugins to v9/v10+ and unfortunately it appears that they switched to ESM in 10.x+, making upgrading a non-trivial task: #5543 (comment)

We simply don't have the team or bandwidth right now to significantly update EuiMarkdownEditor's plugins as a resulting change of this upgrade. We would certainly accept a PR from another team however!

[cee-chen]

I'm continuing the conversation in the linked Kibana thread (elastic/kibana#151276), as that has more context as to full scope and motivation of the request. In the interim I'm closing this issue as a duplicate of #5543.

[watson]

Sorry - I wasn't aware that upgrading would be such a huge workload. I didn't know exactly what these packages was being used for, but looking at their changelog I figured it wouldn't be hard. Fine with closing this issue 👍

[cee-chen]

All good! I thought that too in my original investigation, and got somewhat far (working imports/types etc.), until it came time to open up EuiMarkdownEditor and the entire component was broken 😅 To be honest, it may not be a bad idea in the future for another team (e.g. within Kibana) to take over the markdown editor completely, which would include rewriting it with remark/unified v10+ at that point.