Add support for GitLab Flavored Markdown (GLFM) in VS Code / WEB IDE
### Problem to solve
When editing markdown files in the Web IDE it supports [GitLab Flavored Markdown (GLFM)](https://docs.gitlab.com/ee/user/markdown.html)
in the `Preview Markdown` view.\
This is not the case, when I edit the same file locally with VS Code und use the [Markdown preview](https://code.visualstudio.com/Docs/languages/markdown#_markdown-preview).
The [GitHub Markdown Preview](https://marketplace.visualstudio.com/items?itemName=bierner.github-markdown-preview) extension pack covers many
of the additional GitLab features. But they are designed for [GitHub flavored Markdown](https://github.github.com/gfm/) and do not cover all the
GitLab styles (e.g. `[[_TOC_]]` or the math syntax).
### Proposal
Add functionality to this extension to change the VS Code markdown preview to match the rendering result from GitLab.com.
As you [plan to replace the actual web IDE with VS Code](https://about.gitlab.com/blog/2022/05/23/the-future-of-the-gitlab-web-ide/) for me it seems to be essential
to support GitLab flavored Markdown in VS Code preview.
### Links / references
- https://docs.gitlab.com/ee/user/markdown.html
- https://code.visualstudio.com/Docs/languages/markdown#_markdown-preview
- https://about.gitlab.com/blog/2022/05/23/the-future-of-the-gitlab-web-ide/
- [Add toggle between legacy and new Web IDE](https://gitlab.com/gitlab-org/gitlab/-/issues/370139)
- https://gitlab.com/gitlab-org/gitlab/-/issues/363608
epic