Adds table.footer and grid.footer to have some rows repeat at the bottom of a table's pages.

First part of the sixth task in #3001.

Currently, footers must always include the last row of the table.

NOTE: There are some parts of this PR which are direct improvements/fixes for headers, such as the Repeatable enum, which is used to force non-repeated headers to be unbreakable for consistency. If needed for the release, I can try to extract as much as possible into a separate PR, but, ideally, we'd just merge this PR, which, as I see it, is basically ready. :)

User-facing API

• Use table.footer and grid.footer with repeat: true/false (defaults to true) to have optionally repeatable footer rows at the bottom of the table. When there's gutter, the gutter above the footer is repeated too.

#set page(width: auto, height: 15em)
#table(
  columns: 5,
  align: center + horizon,
  table.header(
    table.cell(colspan: 5)[*Cool Zone*],
    table.cell(stroke: red)[*Name*], table.cell(stroke: aqua)[*Number*], [*Data 1*], [*Data 2*], [*Etc*],
    table.hline(start: 2, end: 3, stroke: yellow)
  ),
  ..range(0, 5).map(i => ([John \##i], table.cell(stroke: green)[123], table.cell(stroke: blue)[456], [789], [?], table.hline(start: 4, end: 5, stroke: red))).flatten(),
  table.footer(
    table.hline(start: 2, end: 3, stroke: yellow),
    table.cell(stroke: red)[*Name*], table.cell(stroke: aqua)[*Number*], [*Data 1*], [*Data 2*], [*Etc*],
    table.cell(colspan: 5)[*Cool Zone*]
  )
)

Implementation details

• Here I mostly just use the same tricks as headers to reduce available space through self.regions.map, use in_last_with_offset etc. This is made possible by the fact that I simulate the footers right before simulating headers, and then skip regions until both header and footer fit. The simulated footer height is kept in the layouter state so rows can be aware of that info, but either way it is subtracted from the available size so rows will properly skip to the next region as needed.
• The footer is only laid out for the current region when finish_region is called, right before effectively resolving rows in lrows, and it only occurs if it hasn't been laid out already, or if it wouldn't be an orphan (and/or the region would only have the header and the footer, in which case both would be orphans and would not be laid out).
• The rest (CellGrid stuff, line priority etc.) is very similar to headers.