Adds support for AbortSignal.reason and related items

[hamishwillee]

AbortSignal has been updated with a reason property that is undefined normally but can be set to any value when the signal is aborted. This PR does the first part of the work of documenting this property and the associated setters.

This is part of the work for #11593

Note, that I am not "done".

• This is no worse than before but it could be improved with some better examples, ideally live examples.
• AbortSignal has another method I will need to add too https://dom.spec.whatwg.org/#dom-abortsignal-throwifaborted
• I don't full understand the cases where you need to set a reason.

[github-actions]

Preview URLs

• https://pr11708.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/AbortController/abort
• https://pr11708.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/AbortSignal/abort
• https://pr11708.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/AbortSignal/aborted
• https://pr11708.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/AbortSignal/reason
• https://pr11708.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/AbortSignal

Flaws

None! 🎉

External URLs

URL: /en-US/docs/Web/API/AbortController/abort
Title: AbortController.abort()
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/AbortSignal
Title: AbortSignal
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/AbortSignal/abort
Title: AbortSignal.abort()
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/AbortSignal/aborted
Title: AbortSignal.aborted
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/AbortSignal/reason
Title: AbortSignal.reason
on GitHub

No new external URLs

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