Finalize spec for link lifecycle

As per this (old) comment from dbaron, we should define steps for the link tag that tie properly into https://html.spec.whatwg.org/multipage/semantics.html#fetching-and-processing-a-resource-from-a-link-element.

We need to check what is already done and what still needs to be done.

At the same time, we should fill the gaps in the lifecycle of the tag, as described in https://docs.google.com/document/d/1-M3cjFGH8EluL9KNMzDcdRzNzSLnc83cRaqudPBRhaI/edit?tab=t.0#bookmark=id.ocoxu518ieg9:

It enumerates its own fetch-and-process triggers (creation, becoming browsing-context connected, and changes to href, disabled, crossorigin or type) and states one MUST: “If a ‘monetization’ link is no longer browsing-context connected, a user agent MUST stop the payment session.”
It does not state what happens to a session already running when one of those triggers fires, and rel is not among the triggers.

Note: for load/error events there are quite a few discussions open in this repo, see https://github.com/WICG/webmonetization/issues?q=state%3Aopen%20label%3A%22link%22. Likely out of scope of this issue, but keep it in mind that this is yet another topic to be tackled.