Add Use Case diagram type

[karollewandowski]

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Proposal

There is a lack of UML Use Case diagrams (or I failed to find it): https://www.lucidchart.com/pages/uml-use-case-diagram

Use Cases

In the Unified Modeling Language (UML), a use case diagram can summarize the details of your system's users (also known as actors) and their interactions with the system

Screenshots

Source: https://www.lucidchart.com/pages/uml-use-case-diagram

Syntax

I have no knowledge of Mermaid conventions, but PlantUML can be a good inspiration: https://plantuml.com/use-case-diagram

Implementation

This is a proposal which I'd love to see built into mermaid by the wonderful community.