PI-11: Add code blocks, quotes, and images to markdown outline

[Copilot]

VS Code's built-in markdown extension only supports headings for document outline. This extends the outline to include fenced code blocks, quote blocks, and images.

Changes

• Standalone parser (markdownParser.ts): Pure TypeScript parser independent of VS Code API

  • Fenced code blocks (``` and ~~~) with language labels
  • Quote blocks (> lines, multi-line aware)
  • Images () with alt text or filename
  • Level 7 hierarchy (below H6) for non-heading symbols

• Enhanced MarkdownOutlineProvider: Combines VS Code headings with custom parser output, nesting additional symbols under their parent headings

• TDD test suite: Unit tests for parser (VS Code independent) + integration tests for provider

Example

# Getting Started

```bash
npm install

Important: Run from project root

Produces outline:

▼ Getting Started
Code: bash
Quote: Important: Run from project root
Image: Screenshot

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by firewall rules:
>
> - `vscode.download.prss.microsoft.com`
>   - Triggering command: `/usr/local/bin/node node /home/REDACTED/work/outline-eclipsed/outline-eclipsed/node_modules/.bin/vscode-test` (dns block)
>
> If you need me to access, download, or install something from one of these locations, you can either:
>
> - Configure [Actions setup steps](https://gh.io/copilot/actions-setup-steps) to set up my environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this repository's [Copilot coding agent settings](https://github.com/doughgle/outline-eclipsed/settings/copilot/coding_agent) (admins only)
>
> </details>

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<details>

<summary>Original prompt</summary>

> plan roadmap item PI-11: add addition symbols for markdown e.g. code blocks, quotes. first check vscode api and built-in markdown language support to understand what symbols are available besides headings. feed back on those.
> 
> if only headings are supported, sketch a solution that will extend the built-in markdown parser to include code blocks and quote blocks and images.
> 
> use TDD approach that unit tests the solution. it should be testable independently of the vscode api. simply taking markdown text lines as input and parsing. create a separate integration test if needed to collaborate with vscode api.


</details>



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