Create fit-spans option

[PgBiel]

Helps with (potentially closes) #56 and #78.

The idea is to be able to specify #tablex(fit-spans: (y: true)) in order to have colspans not affect auto columns' sizes.
That way, fractional columns won't be "squashed".

Consider the table below, from #78:

#tablex(
    columns: (1fr, 1fr, auto, auto, auto),
    [a], [b], [c], [d], [e],
    cellx(colspan: 5)[#lorem(5)],
    [a], [b], [c], [d], [e],
    cellx(colspan: 2)[#lorem(10)], none, none, none,
    [a], [b], [c], [d], [e],
    cellx(colspan: 3)[#lorem(15)], none, none,
)

The last row has a colspan which spans not only the two fractional columns, but also one auto column, causing the latter to expand more than it should (since auto columns are measured before fractional columns), leading to zero-width fractional columns:

Adding fit-spans: (x: true) causes the auto column to ignore the colspan's size, fixing the problem: