fix(revealjs): add aria-label to slide-menu button link (#14249)

The bundled reveal.js-menu plugin creates the slide menu button as
an anchor element with no text content or aria-label, which fails
the axe-core link-name rule (WCAG 2.4.4, Level A). Add aria-label
attribute so screen readers announce the link purpose.

Author: mcanouil

news/changelog-1.10.md

@@ -1,3 +1,7 @@
 All changes included in 1.10:
 
+## Formats
 
+### `revealjs`
+
+- ([#14249](https://github.com/quarto-dev/quarto-cli/issues/14249)): Add `aria-label` to slide-menu button link for screen reader accessibility.