Insert SVG file as `<object>` instead of `<img>` in HTML (better for accessibility)

This is possible in knitr following discussion at https://github.com/yihui/knitr/issues/2152 by opting in using option options(knitr.svg.object = TRUE). Unfortunately, this does not work in Quarto because we don't use knitr plot hooks.

This is important for accessibility as explained by @jooyoungseo at https://github.com/yihui/knitr/issues/2152#issuecomment-1218920811.

When embed-resources: true, <svg> will be included but when embed-resources: false, <img> will be used.

Since svg is embedded via tag, there is no way to access the svg DOM tree within the generated html output. In order to let assistive technology agents, such as screen readers, interact with the svg, we may want to take different approaches.

This treatment for SVG file was discussed in Pandoc at

• https://github.com/jgm/pandoc/issues/7906

but solution recommended there was too use a Lua filter to create the <object> when this is a svg file.

I suggest we do the same in our Lua filter in Quarto.

cc @jooyoungseo

Start by locating Quarto's Lua filter for SVG handling and trace the path used when embed-resources is false; compare the proposed behavior with the linked Pandoc and knitr discussions. Done means non-embedded SVG output uses an object element for accessibility, while embedded resources continue to produce inline SVG as described.