[Feature] Support preview based on configured file type rather than file extension

[moni-dips]

I create a new text file in Code, select type as Mermaid and open the mermaid preview.
Then I click inside the text file and paste some mermaid code.
As troubleshooting steps, I tried moving code to each of my three monitors, but nothing changed.

Expected behaviour

Preview works

sequenceDiagram
    Alice->>+John: Hello John, how are you?
    Alice->>+John: John, can you hear me?
    John-->>-Alice: Hi Alice, I can hear you!
    John-->>-Alice: I feel great!```

### Actual behaviour
Preview does not work

Include:
   * Screenshot ![image](https://github.com/vstirbu/vscode-mermaid-preview/assets/126574585/59ac9322-166b-4aad-a5d6-94bd91ae9f68)
   * Preview mode: Mermaid
   * Theme used: dark

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[moni-dips]

[vstirbu]

The extensions is active for stand alone files with extension .mmd. It does not look like that is the case in the attached screenshot.

[moni-dips]

I just created a new scratch file in code and selected the filetype as mermaid, there is no real file

[vstirbu]

The extension works only with stand alone markdown .mmd files or markdown files with embedded diagrams as describe in the documentation.

Activating the extension based on the file type is not supported. I'll have to investigate how that works before adding that functionality.