lib.network: ipv6 parser from string

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.

lib/network/default.nix

@@ -0,0 +1,49 @@
+{ lib }:
+let
+  inherit (import ./internal.nix { inherit lib; }) _ipv6;
+in
+{
+  ipv6 = {
+    /**
+      Creates an `IPv6Address` object from an IPv6 address as a string. If
+      the prefix length is omitted, it defaults to 64. The parser is limited
+      to the first two versions of IPv6 addresses addressed in RFC 4291.
+      The form "x:x:x:x:x:x:d.d.d.d" is not yet implemented. Addresses are
+      NOT compressed, so they are not always the same as the canonical text
+      representation of IPv6 addresses defined in RFC 5952.
+
+      # Type
+
+      ```
+      fromString :: String -> IPv6Address
+      ```
+
+      # Examples
+
+      ```nix
+      fromString "2001:DB8::ffff/32"
+      => {
+        address = "2001:db8:0:0:0:0:0:ffff";
+        prefixLength = 32;
+      }
+      ```
+
+      # Arguments
+
+      - [addr] An IPv6 address with optional prefix length.
+    */
+    fromString =
+      addr:
+      let
+        splittedAddr = _ipv6.split addr;
+
+        addrInternal = splittedAddr.address;
+        prefixLength = splittedAddr.prefixLength;
+
+        address = _ipv6.toStringFromExpandedIp addrInternal;
+      in
+      {
+        inherit address prefixLength;
+      };
+  };
+}