ENH: Word count for Quarto VS Code Extension

The Microsoft word count extension for VS Code is more of an example extension, despite its large install base and use. Since Quarto is often used for writing documents, it would be nice to have functional and improved word counts available as a standard feature for qmd documents in its extension.

A basic feature list could look like this:

1. Overall document word count
   • Exclusions: may warrant discussion, but it seems like YAML should be excluded and perhaps code blocks.
   • Includes: references (would need to be calculated or at least approximated). For example, I'm writing a book chapter now, and my specs include a word count for references.
2. Selection word count.
3. Section word count. It would be nice to have word counts by section that count everything nested under that section heading. It may be possible to do this in a outline pane or maybe as a popover to the word count in the status bar.

The issue names no files, tests, or entry points; start by locating the Quarto VS Code extension’s handling of qmd documents and status-bar counts. Define and validate overall, selection, and nested section counts, including agreed treatment of YAML, code blocks, and references.