Support plantuml sequence diagram syntax

[chipbite]

Proposal

Mermaid.js should support plantuml syntax since it is one of the most used (if not most) sequence diagram tools and interop would be very valuable for many, enabling use cases such as:

• Trying something out in mermaid, then pasting it into a file for use in a docs setup using plantuml.
• Copying snippets from larger diagrams from a repo, to use in places in github.
  etc

Today, there are several client side sequence diagram tools, some open, some closed (https://sequencediagram.org/ is for instance really nice but not open source in any way). Most (including mermaid) have settled on a syntax like plantuml and others. But there are often slight differences. Most of the time, I suspect the reasons for them are purely historic, there are not many apparent benefits from ->> vs -> or similar differences.

Anyway, thanks for a great tool and I hope that this idea sounds interesting!

Search, found this earlier related issue that was a bit general and unclear - and closed: #149, "Support PlantUML syntax".
I also found that seqencediagram syntax choice was discussed here: #16 (comment)

Example

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Screenshots

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