fix(input[email]): improve email address validation

**Limit size of local-part and total path size in eMail addresses**

RFC 5321 §4.5.3.1.1 ⇒ local-part can have up to 64 octets
RFC 5321 §4.5.3.1.3 ⇒ path “including the punctuation and
element separators” can have up to 256 octets
RFC 5321 §4.1.2 specifies path as ‘<’ + mailbox¹ + ‘>’ in
the best case, leaving us 254 octets

The limitation of the total path size to 254 octets leaves
at most 252 octets (one local-part, one ‘@’) for the domain,
which means we don’t need to explicitly check the domain
size any more (removing the assertion after the ‘@’).

① RFC 821/5321 “mailbox” is the same as RFC 822 “addr-spec”

**Optimise eMail address regex for speed**

There is no need to make it case-insensitive; the local-part
already catches the entire range, and the host part is easily
done. Furthermore, this makes the regex locale-independent,
avoiding issues with e.g. turkish case conversions.

cf. http://www.mirbsd.org/cvs.cgi/contrib/hosted/tg/mailfrom.php?rev=HEAD

**Limit eMail address total host part length**

RFC 1035 §2.3.4 imposes a maximum length for any DNS object;
RFC 5321 §2.3.5 references this (and requires FQDNs, but there
have been cases where a TLD had an MX RR and thus eMail addresses
like “localpart@tld” are valid).

Credits: Natureshadow <d.george@tarent.de>

**Limit eMail address individual host part length**

A “label” (each of the things between the dots (‘.’) after the ‘@’ in
the eMail address) MUST NOT be longer than 63 characters.

cf. http://www.mirbsd.org/cvs.cgi/contrib/hosted/tg/mailfrom.php?rev=HEAD
and RFC 1035 §2.3.4

**Fix eMail address local-part validation**

A period (‘.’) may not begin or end a local-part

cf. http://www.mirbsd.org/cvs.cgi/contrib/hosted/tg/mailfrom.php?rev=HEAD
and RFC 822 / 5321

Closes #14719