[NEW] Quotation mark styles for different languages

[Rob Beezer]

Elements related to quotations will now react to the language given by @xml:lang
attributes. More precisely, #q (primary quotation group) and #sq (secondary
quotation group), will use marks according to the closest enclosing element with
an @xml:lang attribute. The four rarely-used elements that produce right/left
single versions of the marks will also behave similarly.

See the sample article (especially in PDF form!) at the end of:

Subsection 9.2: Quotations
https://pretextbook.org/examples/sample-article/html/section-text-paragraphs.html#quotations

Notes:

* There is no publisher switch for this, no opt-in or opt-out. Changes will
just roll-out with updates.

* You can always put a @xml:lang attribute directly on a #q or #sq for a
one-off change.

* This affects outputs to HTML, EPUB, Jupyter, and LaTeX. Braille is
out-of-scope, too lazy to fixup the conversion to ASCII text right now, and
WeBWorK is not set-up to digest this.

Caveat: only implemented for en-US and fr-FR at the moment. Yes, I picked
up the thin space. fr-CA will be trivial to turn on next. I may post here as
new languages receive an implementation.

Feedback: I'd love to hear from authors of projects in French about how this is
working for them. Canadians can briefly turn on the French attribute for
testing? You'll need the CLI nightly, or the pretext/pretext script.

Plans: any scheme with marks in Unicode and as LaTeX macros is a candidate for
implementation. Like quote marks down at the baseline? Corner-brackets for CJK?

Language maintainers
~~~~~~~~~~~~~~~~~~~~

* Documentation is in the en-US file, as usual.

* This is accomplished with two mandatory attributes on the #locale element.
If a scheme/variant/style is available already that fits your language, it is a
simple edit. If we need a new style it is a bit more work, but easy for me to
do. For now, every file (other than fr-FR) has English quotes specified as a
default.

* More important is to help me understand what your language uses. Since we
are producing scholarly documents, more formal versions are preferred. (In
other words, maybe the custom is to use English marks online, but we would opt
instead for whatever a national style guide says, or whatever custom is used in
research papers and textbooks.)

* I'm putting a test of each new quotation style (not each new language) in the
sample article with some iconic quotes (link above). You can help with that, too.

* Chevrons/Angles, without a thin space, will be the easiest to do next.
Italian? Spanish (Spain/Mexico/LatAm?)?

* The pretext-dev group is a good place to start a discussion.

Rob