tex-newtx{,-doc}: update to 1.554

Version 1.55 adds support for the SticksToo text fonts (a reworking of
STIX2) as option stix2 to newtxmath. Along with this addition, there
are some important code differences concerning the treatment of
embedded Blackboard Bold alphabets, and, unique to the behavior under
stix2, the addition of the built-in DSSerif alphabet to the options for
Blackboard Bold alphabet.

(markd)