Warning likely to almost always appear

[breezykermo]

Since #77, there is now likely to be a warning on most watch and compile commands that looks something like this:

warning: unknown font family: helvetica
   ┌─ index.typ:29:17
   │
29 │ #set text(font: ("Times New Roman", "Arial", "Helvetica"))
   │                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The issue here is that we provide fallback fonts in order to ensure Rheo renders reasonably, but the Typst compiler complains if any of the fonts do not exist.

One solution to this would be to suppress this warning in Rheo (like we do with the 'HTML experimental' warning). Perhaps a more robust solution would be to embed a default font such as Times New Roman in the Rheo CLI, so that we are not relying on variable state (the presence of certain fonts) in the user's system.