chore: create eest (& eels) label concept for use in execution-specs post-weld

[danceratopz]

We will transfer all relevant issues to execution-specs At-Switch. Beforehand we should clean-up our labels and align their style to those in execution-specs, so that we have a unified label concept post-Weld.

Create label concept: EELS and EEST have differing label concepts and these should be unified for good repo hygiene post-Weld. Currently, only priority labels have been aligned (although EELS misses P-Urgent, I believe). As a team we agreed that we would adopt EELS label standards post-Merge; but they will need expanding and perhaps some other modifications. One large part of this task is to create reasonable set of EEST scope:* labels as we have far too many of these.

This concept should be discussed in the team.

[SamWilsn]

The principles are:

• Labels are grouped.
• Each group has a common prefix.
• Each group has the same color.
• At most one label from each group is added to each issue/pull request (this is fast-and-loose; if something touches multiple areas, it deserves multiple A-* labels).

There's a little more information over at: https://forge.rust-lang.org/release/issue-triaging.html

[kclowes]

Once the label concept is decided, we can script the label transformations through the gh cli. I have a basic prototype working that I can either run or push up for someone else to run, whatever is easiest!