Add RStudio-style cell/chunk dropdown selector for `.qmd` / `.Rmd`

System details: Apple M4, 24 GB, Sequoia 15.2

Positron and OS details: Positron Version: 2025.07.0 (Universal) build 204

Code - OSS Version: 1.100.3
Commit: 03ae7a5393c776bb24c23d2aa6a6bfbba90cbc5e
Date: 2025-06-30T16:28:59.945Z
Electron: 34.5.1
Chromium: 132.0.6834.210
Node.js: 20.19.0
V8: 13.2.152.41-electron.0
OS: Darwin arm64 24.5.0

Session details:

Running R 4.5.1

Describe the issue:

Data Explorer Enhancement
When displaying Rmd files in the explorer the selectable data chunk list that is available in RStudio is not present in Positron. I make extensive use of this chunk dropdown list to locate sections of the code of interest. In the screen shot example (inset below) a notes.Rmd file is divided into chunks making it easy to quickly locate and jump to specific notes.

Steps to reproduce the issue:

1. Simply load any Rmd file into the positron data explorer
2. Notice that the chunk dropdown selector found in RStudio is not available in positron.

Expected or desired behavior:

As seen in RStudio

Were there any error messages in the UI, Output panel, or Developer Tools console?

No error as this function was omitted in positron

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Start by loading an Rmd or qmd file in Positron's data explorer and compare its navigation controls with the RStudio example linked in the issue. Identify the existing explorer entry point and any related tests; done means users can select and jump between document chunks through a dropdown.