Scribble: add 'grouper and 'hidden-number part style properties

Also, clean up documentation and implementation of 'unnumbered and
'hidden.
Scribble: add `scribble/book' and `scribble/report' languages

These languages set the default class for Latex rendering, and also
adjust Latex commands so that top-level sections are chapters, etc.
fix prototype when the C compiler has no "no inline" annotation
Scribble: fix docs on Latex commands for parts and part references
fix `raco link' test
work around C library bug in powl()
racket/date: change `current-date', add `date*->seconds'
avoid a deadlock introduced in recent identifier highlighting commit
racket/gui: doc clarifications on refreshes and edit sequences
reference: fix typo and terminology for `with-continuation-mark'

Closes PR 13478
another close-paren insertion repair

(this one causes runtime errors, not
bad behavior)
Fix `sequence-lift` for multiple valued sequences.

Also minor fixups of the rest of `unstable/sequence`.
fix a problem with the way that check syntax's interval map
tracks information about the program
x86_64-mac: Pango CoreText repair

Closes PR 13473
fix doc mistake

Closes PR 13482
fix doc typo

Closes PR 13483
fix find-seconds error reporting to say "find-seconds" instead
of "find-secs"
change the default test coverage colors

After some consultation with my class this quarter,
it seems clear that the previous defaults and nearby
variations are completely hopeless for colorblind people
(there are at least 3 in this class). So go with the
defaults used in the HtDP languages, which have been
vetted by saidsame 3 people.
give up on the "find the close paren strategy that doesn't
edit the buffer" attempt

In other words, go back to (a rackety version of) the old
implementation of the get-close-paren method
Spelling error

Also, Rackety