V4: Mermaid Codeblocks -- Add Class Diagrams

[Anarchtism]

Is your feature request related to a problem? Please describe.
Mermaid Codeblocks uses flow charts which doesn't show which direction is up, down, prev, or next.

Describe the solution you'd like
I'd like to be able to use Class Diagrams in Mermaid instead of flow charts. Specifically so it can show arrowhead styles based on the relationship.

classDiagram
    class `Test`
    class `Up of test`
    class `Down of test`
    class `Prev of test`
    class `Next of test`
    `Test` o--* `Up of test`
    `Test` *--o `Down of test`
    `Test` <|--> `Next of test`
    `Test` <--|> `Prev of test`

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