Allow `rowspan` and `colspan` to take `auto`

[platformer]

Description

Passing auto to rowspan or colspan would force the cell to take as much space as possible. For example:

#tablex(
  columns: 10,
  [a], colspanx(auto)[b], [c]
)

The b cell would span 8 columns.

Notes

I'm not sure what should happen if there are multiple automatically-sized spans in the same row/column. Either the available space could be divided evenly among each such cell, or it could be outright disallowed.

[PgBiel]

Could also play with the idea of allowing 1fr, 2fr syntax.

[PgBiel]

I am, however, unsure if this is feasible for rows, as theoretically there are unlimited rows. Could perhaps restrict that to a single page.