Move /welcome screen to be within extension

[sidvishnoi]

https://webmonetization.org/welcome/ is used as interact.finish screen as OP cannot open a browser internal page on grant finish. Given that page exists (welcome.astro in WICG repo), we decided to add there status messages (success/failure) for various scenarios on grant completion.

But we can provide any URL there, intercept it via extension, and then redirect to an internal page.

Having internal page has benefits:

• Keep extension concerns separate from website.
• Keep different error scenarios concentrated within extension.
• Not require any versioning logic like we intended to add to /welcome page, keeping the code and flow clean welcome.astro#L150.
• Use extension styles instead of trying to make them work on website.
• Support message localization using native extension features. #979
• Show more details with errors by message passing (query parameters can only do so much on existing welcome page).
• Track error/success rate/reasons #369