VS Code extension: Rmd does not export symbols

The Quarto extension fails to provide symbols for an R Markdown document such as scratch.Rmd (content below). The issue prevents:

• the breadcrumbs from being shown
• the outline from being displayed
• the Jump to symbol navigation shortcut box being populated

This occurs in
Positron Version: 2024.07.0 (Universal) build 21
Code - OSS Version: 1.90.0
Commit: 82b335e27545e804b7fd806db00477a6eb49951d
Date: 2024-07-09T04:32:26.804Z
Electron: 29.4.0
Chromium: 122.0.6261.156
Node.js: 20.9.0
V8: 12.2.281.27-electron.0
OS: Darwin arm64 23.5.0

Also in Quarto v1.113.0 in VS Code

If I install REditorSupport.r extension then the R Markdown document symbols appear as normal, but not in the Quarto extension.

This issue was previously raised as Positron Issue #3982 and now transcribed here at the request of @juliasilge

Code for scratch.Rmd for testing is below:

---
title: "A wonderful R Markdown document"
output:
  html_document:
    math_method: katex
---

```{r echo = FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

## R Markdown

This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see <http://rmarkdown.rstudio.com>.

Here is some math:

$1 + 1$

When you render this file, a document will be generated that includes both content as well as the output of any embedded R code chunks within the document. You can embed an R code chunk like this:

```{r cars}
summary(cars) 
```

## Including Plots

You can also embed plots, for example:

```{r pressure, echo=FALSE}
plot(pressure)
```

Note that the `echo = FALSE` parameter was added to the code chunk to prevent printing of the R code that generated the plot.

Start by reproducing the missing symbols in scratch.Rmd with the Quarto extension and compare the result with the REditorSupport.r extension. Trace the Quarto extension's symbol handling, then verify that breadcrumbs, the outline, and Jump to Symbol are populated for the supplied document.