Add a library function to parse and validate an IPv6 address from a
string. It can parse the first two versions of an IPv6 address according
to https://datatracker.ietf.org/doc/html/rfc4291#section-2.2. The third
form "x❌x❌x:x.d.d.d.d" is not yet implemented. Optionally parser can accept prefix length (128 is default).
Add shell script network.sh to test IPv6 parser functionality.
