Breakable tables

[Rob Beezer]

OK, there is some *preliminary* support now for breakable tables.

Short answer: set @break="yes" on a "tabular".

Caveats:

* Only for LaTeX conversion to PDF, not really relevant otherwise.

* Not just for long tables, a short table could break at the bottom of a page
if there is not enough room.

* Do not use in a side-by-side panel, seems LaTeX compilation fails.

* Some features are not behaving, like horizontal rules and maybe paragraph
cells. Start simple and then ramp up. Reports of additions that first provoke
damage are welcome, especially on the issue below. No promises that they will
be addressed.

https://github.com/PreTeXtBook/pretext/issues/1803

* See good and bad examples in "Table Calisthentics" in sample article.

* Sean sent me his provoking example off-list. Final (of four) columns ended
up in the margin. I suspect this is becauase every column held paragraph cells,
which is a demanding feature.

* Minimal documentation and warnings in the Guide.

* Only announcing on pretext-dev so experienced developers and authors can
explore this first.

Implementation:

1. Swap in the "longtable" package when breakability is requested.

2. Too complicated to have a second tcolorbox style for "table", so just
added "breakable" to the existing base style. Should not have any real effect
on existing projects since a LaTeX "table" is not breakable anyway, just a
"longtable" is.