Request: PlantUML parser format

[sevenjay]

support PlantUML parser format like below

```plantuml
@startuml
[*] --> Initialed
Initialed --> Ready

```plantuml
[*] --> Initialed
Initialed --> Ready

keep @startuml or not is fine,
and other non-UML diagrams are also supported

```plantuml
@startjson
{
   "fruit":"Apple",
   "size":"Large",
   "color": ["Red", "Green"]
}
@endjson

There are many tools support this format https://notepm.jp/blog/6589
The context after only @startuml will be parser as markdown. use ```plantuml will parser it as code

[yuki-takei]

related issue: #6074

[yuki-takei]

Update

#6604

Please wait GROWI v6 which will be released the winter 2022.