🧮 Give title/description to math macros

[fwkoch]

You may now define title/description for math macros in frontmatter, e.g.

math:
  '\aMat':
    title: Custom Macro
    description: Macro I made up only for the sake of this example
    macro: '\mathrm{a}'

Defining a simple command:macro pair is also still valid:

math:
  '\aMat': '\mathrm{a}'

This PR also updates the typst exports to resolve recursive math commands prior to filling them in to the document. Latex renderers handle recursive commands just fine, but typst does not so we must manually resolve them early.

Finally, this makes one small change to simple-validators where validateObject no longer validates arrays (these still have typeof 'object' so they passed before). Only allowing actual Record<string, any>-style objects has always been the expected behaviour, so I consider this a bug fix as opposed to a breaking change.

[rowanc1]

Looking through, looks good overall, and excited to see these improvements!