Change the name and return type of
`IPv6Address::to_deprecated_string()` to `IPv6Address::to_string()`
with return type `ErrorOr<String>`.
It will now propagate errors that occur when writing to the
StringBuilder.
There are two users of `to_deprecated_string()` that now use
`to_string()`:
1. `Formatted<IPv6Address>`: it now propagates errors.
2. `inet_ntop`: it now sets errno to ENOMEM and returns.
This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.
One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.
This is the IPv6 counter part to the IPv4Address class and implements
parsing strings into a in6_addr and formatting one as a string. It
supports the address compression scheme as well as IPv4 mapped
addresses.
