Add preamble for custom typst code

[cgahr]

Some backends like pgf/latex backend support preambles
#pgf.preamble: # See text.latex.preamble for documentation
It would be nice to support them as well for typst.

For example, I use

#let phi = math.phi.alt

to display the phi that I want without typing phi.alt. If I write $phi$ in i.e. labels, it does however display the standard phi and not the one that I want.