For #140602
Only normal links for now. Will add reference links later. Should support the forms:
- `[](dir/file.md)`
- `[](./dir/file.md)`
- `[](/root-dir/file.md)`
- `[](#header)`
- `[](./dir/file.md#header)`
* Fix for #26659.
Clicking on a local file link will open up the editor on a separate editor group (new or reuse existing one).
* Fix for #26659: Add way to open Markdown links in a different editor group
Adding "markdown.editor.openMarkdownLinks" setting to specify where
links to markdown files should open (current editor group by default).
Switch to always hiding the frontmatter and then allowing markdown-it plugins to render it properly if they wish. `previewFrontMatter: "show"` is also not very useful since it usually results in a jumble of text at the top of the file
This is required with the new performance work to avoid re-tokenizing the document multiple times during rendering
