Centralize blocked port list

[annevk]

fetch/api/request/request-bad-port.html and websockets/Create-blocked-port.any.js both include the list of ports at https://fetch.spec.whatwg.org/#port-blocking. Perhaps we should store this list separately so it can be shared. (The one problem here is that if we store it as JSON and later the blocking logic becomes more complex (e.g., either in a range or in the list), we'll have to revamp the setup again.

Also sharing it with tools/wptserve/wptserve/utils.py is probably not a good idea, but I'm willing to be convinced otherwise.

[annevk]

(Suggested in #11249 by @Ms2ger.)