Markdown Preview Mermaid Support

Adds Mermaid diagram and flowchart support to VS Code's builtin markdown preview

Usage

Create diagrams in markdown using mermaid code blocks:

```mermaid
graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;
```

Mermaid

Currently supports Mermaid version 8.4.8.

Add custom CSS support

You can use the built-in functionality to add custom CSS. More info can be found in the markdown.styles documentation

For example, add Font Awesome like this:

"markdown.styles": [
    "https://use.fontawesome.com/releases/v5.7.1/css/all.css"
]

Use it like this:

```mermaid
graph LR
    fa:fa-check-->fa:fa-coffee
```