curly-injectors to target parent "page" element if no preceding sibling target

If the {curly-injector} appears as the first chunk on a page (i.e. very beginning of brew, or following a \page) then have the values injected into the enclosing <div class="page V3"> element.

That is, instead of defaulting to treating the {curly-injector} as plain text.

---

Example: adding a class to a page:

\page
{songsheet}
Non, rien de rien
Non, je ne regrette rien
Ni le bien, qu'on m'a fait
Ni le mal, tout ça m'est bien égal

Note that the usual classes of “page” and “V3” should be retained, even in face of {class="overwrite"}.

---

Example: adding a lang attribute to a page:

\page
{lang=fr}
Non, rien de rien
Non, je ne regrette rien
Ni le bien, qu'on m'a fait
Ni le mal, tout ça m'est bien égal

(note that adding a lang attribute to the whole brew is a separate issue #1343, and separately needed.) (note 2: here we are injecting a html-attribute, not a style class or style rule. See #1489 for that.)

An attempt to override the id of a <div class="page" id="p123"> should be ignored.

[calculuschild]

Right after \page seems more logical syntax to me. Injector curlies already can be used to target the immediately-preceding element. This expands the syntax to apply to the immediate parent if there is nothing preceding. This could apply to other nesting structures, for instance table cells? Divs?

|  header |  header |
|:-------:|:-------:|
|{blue} hi| not blue|

{{A div
{blue}
content?
}}

Not that we want to implement any of those necessarily.

But my main concern is so far none of our other syntax uses “keywords” to trigger something except for \page and \column, and I’m hoping we can keep those special cases to a minimum. The pagenumber syntax uses pageNumber as a CSS class, but not a functional keyword, if that makes sense. It can be placed anywhere because it’s a standard DIV with an absolute position, so it doesn’t matter where in the markdown it goes.

That’s my 2 cents. I prefer expanding the syntax over adding keywords.

[Gazook89]

Closing this per above. It can be re-opened if we really still need to be able to inject into parent elements directly.