Re: `:export' attribute?: Re: Experimental public branch for inline special blocks

["Juan Manuel Macías" <maciaschain@posteo.net>]

To summarize the latest improvements and modifications to the `:export'
attribute...

The attribute supports one or more elements separated by a space. Each
element can be any of the following signs: "*" (export only the
content), "-" (do not export), "=" (export the rest normally), "=*"
(export the rest, but only the content), "=-" (do not export the rest).
Additionally, backend names can be given explicitly, alone or
accompanied by the "*" or "-" signs, that is (where "backend" equals the
name of the backend):

  backend: export normally for that backend and its derived backends;

  backend-: do not export for that backend or its derived backends.

  backend*: export only the content for that backend and its derived
  backends.

Some examples with valid combinations:

@foo[:export *]{lorem ipsum}

==> export only the content

@foo[:export -]{lorem ipsum}

==> do not export

@foo[:export latex-]{lorem ipsum}

==> do not export to LaTeX

@foo[:export latex- html* =]{lorem ipsum}

==> do not export to LaTeX, export only the content to html and export the
rest normally.

@foo[:export latex =*]{lorem ipsum}

==> export normally to LaTeX but export only the content to the rest

@foo[:export latex =-]{lorem ipsum}

==> export normally to LaTeX and not export to the rest

And below is a complete example based on a possible use case that Max
had exposed:

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
 To see a demo of the Fairlight CMI, watch @@[:export html =-]{this
 video} @@[:export
 html-]{[[https://www.youtube.com/watch?v=am0GBuS_7cE][this video]]}
 with Peter Vogel.
 
 #+begin_export html
 <iframe width="560" height="315"
 src="https://www.youtube.com/embed/am0GBuS_7cE?si=X7pghJhtdfOT1hby"
 title="YouTube video player"
 frameborder="0"
 allow="accelerometer;
 autoplay; clipboard-write; encrypted-media; gyroscope;
 picture-in-picture; web-share"
 allowfullscreen></iframe>
 #+end_export
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━