Quarto Outline: option to show only document headings like RStudio

For large `.qmd` files, the Outline view includes all symbols/objects defined inside R code cells in addition to the Quarto headings. This makes the outline very noisy and can also make it slow to load. It would be useful to have an option to display **only Quarto/Markdown headings (`#`, `##`, `###`, etc.)** in the Outline, similar to the document outline available in RStudio.