[add] support for non-numerical content in aligned cells

[Mc-Zen]

This PR adds support to include some additional content in a table cell containing a numeral while keeping the ability of aligning the numeral to the decimal marker (and more).

This adds a new function nonum() that can be used to mark content to be protected from number recognition.

#ztable(
  format: (auto,),
  [#nonum[€]123.0#nonum(footnote[A special number])],
  [222.23+-2],
  [12],
)

Fixes #7