[Editorial] Rename 'payment handler' to 'web-based payment handler' in the spec text

[stephenmcgruer]

closes #???

The following tasks have been completed:

• web platform tests (link)
• MDN Docs added (link)

Implementation commitment:

• Safari (link to issue)
• Chrome (link to issue)
• Firefox (link to issue)
• Edge (public signal)

[stephenmcgruer]

I renamed almost all of the payment handler mentions to web-based payment handler, whether or not they were a link or just text. It is possible that some of them either (1) should be link-ified and/or (2) should be named back to just 'payment handler' (e.g., if they're talking about the generic concept). Feel free to suggest places that should be changed :)

[ianbjacobs]

(I deleted my comment; I left questions on a different pull request.)