Use LuaLaTeX for better microtype support

[rileylev]

The microtype package (https://ctan.org/tex-archive/macros/latex/contrib/microtype) implements microtypographical optimizations that can make the body of the text look more professional and easier to read. However, most of these optimizations require LuaLaTeX instead of XeTeX. We still keep fontspec and all.

[lhf]

Hear, hear!