Introduce `BoxMarker` to pretty-printing.

The pretty-printers open and close "boxes" of text a lot. The open and
close operations must be matched. The matching is currently all implicit
and very easy to get wrong. (#140280 and #140246 are two recent
pretty-printing fixes that both involved unclosed boxes.)

This commit introduces `BoxMarker`, a marker type that represents an
open box. It makes box opening/closing explicit, which makes it much
easier to understand and harder to get wrong.

The commit also removes many comments are on `end` calls saying things
like "end outer head-block", "Close the outer-box". These demonstrate
how confusing the implicit approach was, but aren't necessary any more.

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