Support for standalone .mmd or .mermaid files

[dashdeckers]

It would be nice if .mmd or .mermaid files were automatically recognized and rendered as I am restricted to using those (other tools only support them, and in general it is not a bad design choice to keep large diagrams in separate files) and now I need to have two extensions installed to be able to live-preview my mermaid diagrams: this one and "Mermaid Preview".

These standalone files are not fenced in code blocks like this:

```mermaid
stateDiagram-v2
    s1 --> s2
```

instead they simply contain the contents like this:

stateDiagram-v2
    s1 --> s2

[Kingdruoid]

+1
I am also looking to achieve this exactly.