Implement Thai Font Shaper

[JimBobSquarePants]

Now that #198 has been merged the fundamental infrastructure is there to allow support for Bidi text rendering however we still lack some dedicated shapers. This issue tracks the development of a Thai shaper.

Guide.
https://github.com/n8willis/opentype-shaping-documents/blob/master/opentype-shaping-thai-lao.md

HarfBuzz Implementation
https://github.com/harfbuzz/harfbuzz/blob/main/src/hb-ot-shaper-thai.cc