Add option for Markdown-style hierarchical outline in .Rmd files #1692

For .Rmd files, please add an outline mode that matches standard markdown heading hierarchy in VS Code (for .md files). The problem with the current full outline is that:

1. Every function in every cell, along with normal section headers (#, ##, ###) are all put in the outline, making it difficult to find what you need in large .Rmd files;
2. The sections are not hierarchical. Normal .md files have hierarchical list (folding out using [+]).

Ideally this would be configurable with options, e.g. headings-only, headings+chunks, and full code symbols, so users can keep R Markdown editing features without losing the normal hierarchical document outline.

In the meantime, users can switch off Rmd-support with the plugin by adding the following to settings.json, so that VSCode uses normal markdown outlines for .Rmd files (but no R-plugin functionality either):

{
 "files.associations": {
      "*.Rmd": "markdown"
   }
}

I guess this would need changes in the languageserver implementation?