[add] accessibility for num

[Mc-Zen]

This PR adds support for specifying manual as well as generating automatic alt texts to standalone numbers in equations for export to accessible formats like PDF UA-1.

This is done by adding a parameter alt that can be one of the following

• a string for specifying a manual alt text
• auto for automatic locale-aware description generation (PRs for adding more translations welcome!)
• a dictionary with the keys times, plus, minus, and power. For English, the corresponding values would be "times", "plus", "minus", and "to the power of".
• a function that receives a dictionary with the keys int, frac, decimal-separator, base, expoenent, abd uncertainty.

Work in progress: alt texts for units.

Note that when a num is inserted into an equation (like $a=#num[2.2e4]$) the alt text of the number will be dismissed since the equation is merged with the outer equation. Maybe Typst will add some modularity in the future.

Closes #82