tinyxml2 2.0.2: new formula

tinyxml2 is a light-weight c++ library for parsing xml. It
was previously submitted as a head-only formula in #23381
but was put on hold until releases could be tagged upstream.
The 2.0.2 release [has just been tagged](https://github.com/leethomason/tinyxml2/issues/31),
so I think the formula may be considered again for inclusion.