Feature: Allow changing execution order of map-rows and map-cols

[HallerPatrick]

I quickly setup a working modification, such that the user can decide which mapping function is called first and second. I dont know if this is idiomatic typst code. Feel free to adjust or discard :) This solves #123

[PgBiel]

Thanks!
Do you think you can rebase this PR on (and point it to be merged to) 0.1.0-dev, where the latest developments occur? Should be a matter of just moving code to the appropriate files.

If not, I can try to do it at some point.

[PgBiel]

I think it should just be a string, "rows" or "cols". This design would allow running the mappers multiple or even zero times.

[HallerPatrick]

So you mean, if map-order is "cols", then it should first run the cols mapper and then the rows mapper and vice versa? Then I would choose a more descriptive name for the options such as "rows-first" and "cols-first".