internal/parser: add PublicSuffix and RegisteredDomain methods to List

[danderson]

These calculate the public suffix or registered/registerable domain of some input domain, according to the strict PSL algorithm. Aside from being useful as a reference implementation of the algorithm, it comes in handy in validations that need to do stuff like figure out what registry to query for domain info.

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Please check my work carefully on the PSL algorithm, in particular the handling of wildcard domains. I read the spec very carefully and also the platform.sh wiki page, and I think I've correctly implemented the strict PSL algorithm with no deviation (a public suffix *.foo.com does not imply a public suffix foo.com, and there is an implicit hardcoded * public suffix so that all unknown TLDs are automatically public suffixes).

Later on I could add modifiers to these methods to implement the variants as well, but for now I stuck to the reference algorithm. I ended up needing this logic to implement RDAP support for checking domain expiry, since I have to map a random PSL entry to "which registry RDAP service should I query", and for some suffixes that requires computing the "next" public suffix of the PSL entry :)

[wdhdev]

To confirm, a *.foo.com entry would indicate all subdomains at the third level (e.g. bar.foo.com) are declared as TLDs.

[danderson]

TLD is may be a slight misnomer, in that it might imply the involvement of IANA-level registries. But with an adjusted term, yes: *.foo.com means that all 3-label domains ending foo.com are public suffixes in some registry (public or private), and all 4-label domains ending foo.com are registered/registrable domains.

Said in terms of cookie security, a browser will allow setting cookies on fourth.bar.foo.com (registered domain), but not on bar.foo.com (public suffix).