a11y: profvis separator images have no alt attribute (image-alt) #14379

Reported in #14373.

Repro: https://github.com/alexpghayes/quarto-accessibility-reprex (post/post-1)

axe rule: image-alt · impact: critical · tag: wcag2a (1.1.1)
Target: <img class="separator-image" src="data:image/png;base64,...">

The profvis htmlwidget injects decorative separator images in its status bar without an alt attribute, failing the axe image-alt rule. These are purely decorative — alt="" would resolve it.

Root cause is upstream in rstudio/profvis. Tracking this in Quarto so we notice recurrences; a separate issue will be filed at https://github.com/rstudio/profvis.

Part of #8706.