Need to consider how to cancel polling

Four has no way to cancel `hx-trigger="every ..."` polling.

Current state:

| | 2.x | four |
|---|---|---|
| polling | recursive `setTimeout` (`processPolling`) | `setInterval` (`src/htmx.js:830`) |
| cancel state | `internalData.cancelled` | none |
| HTTP 286 | yes | **no** |
| public API | none | none |

2.x honors a `286` response to stop polling, documented at https://htmx.org/docs/#polling
and inherited from intercooler's `X-IC-CancelPolling`. Four drops it silently.

Things to decide:

- Should four honor `286`?
- Should there be a public `htmx.cancelPolling(elt)`?
- Is cancellation permanent, or is there a `resumePolling`?

See #2576 for a 2.x proposal that added three mechanisms at once
(`286`, an `HX-Cancel-Polling` header, and `htmx.config.cancelPollingOnError`).
Corresponding 2.x issue: #2489