Add support for converting the TOC to raw markdown

**Is your feature request related to a problem? Please describe.**
I really like the `[TOC]` feature. Most of my markdown is however meant for use on GitHub, and there, the raw text "[TOC]" is displayed. I always have to copy-paste my entire markdown into an online tool (like [this one](https://luciopaiva.com/markdown-toc/)) to generate a markdown TOC.

**Describe the solution you'd like**
A button or shortcut to insert an actual raw markdown TOC

**Describe alternatives you've considered**
1. When linking to github, the conversion of [TOC] to actual markdown (not ideal)
2. a button or right click menu action to convert the `[TOC]` tag into actual markdown (more ideal)