Add abortPromise as a helper alongside abortSignal

[benjie]

Description

Still experimental; this allows you to await Promise.race([helpers.abortPromise, doTheThing()]) in your jobs, and have the timeout/abort fire conveniently.

Extracted from #474

Performance impact

Negligible - we construct it up top and it's shared by all tasks (not created on a per-task basis).

Security impact

None that I can think of 🤔

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.