UnicodeMath as a submodule

[manuelbb-upb]

Instead of registering UnicodeMath.jl as a new package, it could become part of UnicodeFun.jl perhaps, as it offers similar functionality.

Motivation: consistent plots with MathTeXEngine, see Kolaru/MathTeXEngine.jl#143
But the code felt out of place as part of MathTeXEngine.

For MathTeXEngine I really just need UnicodeMath somewhere, so I have made it a submodule and exported nothing.

But we can easily add some functionality to UnicodeFun as well:

• There are some additional LaTeX commands that are handled by to_latex.
• The styling functions like symbf are recognized as commands in to_latex.
• to_latex can do "normalization", e.g., make all latin letters italic.

Note: UnPack should be easy to remove if you think it an unnecessary dependency.

[manuelbb-upb]

after some tinkering, I think it is better to keep the packages separate