Need to consider how to cancel polling

[1cg]

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

[manwithacat]

RFC on the hypermedia shape of this, rather than restoring 286 / adding cancelPolling: #4048 (draft PR #4049 — docs + tests that pin current four-dev, no new API).