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

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?

Start by investigating the language server implementation and the settings.json files.associations workaround described in the issue. Compare the current .Rmd outline with VS Code's Markdown hierarchy, then verify configurable headings-only, headings+chunks, and full code-symbol modes with hierarchical sections and folding.