Skip CI steps irrelevant to committed changes on PRs

[Gedochao]

Relevant to:

• Optimise & refactor the testing pipeline #2937

This is an attempt to save time on running tests irrelevant to changes on a given pull request.
i.e. when changing a doc or adding release notes, it is essentially pointless to run the whole integration suites, which pessimistically can run for hours, due to all the platforms necessary.

A new changes job is added, which classifies changes to categories. Steps in other jobs are then subsequently skipped based on category.
This is a bit sneaky, as the jobs have to still "run" to satisfy our PR checks. The actual status on what was run can be checked based on the time a job elapsed & what was logged.

main branch & release tags bypass this, retaining the old behaviour (in fact, all jobs which don't originate from a PR retain the old behaviour, running everything).

Additionally, override keywords have been added to allow to still run certain test suites even if the code in a PR would imply they're irrelevant. They're checked in the PR description and have syntax in the style of the Scala 3 compiler repo. Documented them in DEV.md.
Not including them in the PR description, as they would actually change behaviour 🙃

Checklist

• tested the solution locally and it works (can't test locally)
• ran the code formatter (scala-cli fmt .) (irrelevant)
• ran scalafix (./mill -i __.fix) (irrelevant)
• ran reference docs auto-generation (./mill -i 'generate-reference-doc[]'.run) (irrelevant)

How much have your relied on LLM-based tools in this contribution?

extensively, Cursor + Claude

How was the solution tested?

Testing it on the CI, impossible to test locally.

[tgodzik]

LGTM, though it's a lot of changes, so just had a quick look

[Gedochao]

though it's a lot of changes, so just had a quick look

it's also hard to fully test this without getting it on main.
So let's just observe after I merge.