Simplify the public suffix and registrable domain definitions

[rmisev]

As of April 1, 2025, the Public Suffix List algorithm was modified so that the trailing dot of a domain would be retained in the result. Quotes from this algorithm:

• The public suffix is the set of labels from the domain which match the labels of the prevailing rule, using the matching algorithm above. A trailing dot shall be added if the domain included one. (See Note 3)

Note 3
Callers of the algorithm often cannot determine if the domain to be queried is an FQDN but may treat example.com and example.com. as distinct. To support this, the algorithm should retain that distinction. This means looking up example.com should yield com while looking up example.com. should yield com. as the public suffix.

Therefore, there is no need to add a trailing dot to the result if the input domain has one, as this dot is retained by the PSL algorithm.

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[annevk]

Not terribly impressed that the specification is on a wiki now... publicsuffix/list#792 is where the change was discussed.

This PR looks good to me, but I'll wait a bit with merging.

[shannonbooth]

This removes the ASCII assert previously added by c28ff95 - which I think was valuable as an assert as this did clarify behavior for me. But maybe it makes sense to explicitly have a spec note for it that we're using A-label instead of (or in combination of?) an assert?

[rmisev]

I have returned previously added asserts.

[annevk]

Can we keep trailingDot and use it in this assert? Like "publicSuffix is an ASCII string that ends with trailingDot"?

I like the additional clarity that gives especially since the Public Suffix specification is rather informal. Hopefully at some point we define Public Suffix in a proper standards body, but until that day...

We should also link "Assert" now that we're changing this.

[rmisev]

@annevk I made the changes you suggested.