Support curly brackets around code block language/graph type name

[arwedus]

With the popular MyST Markdown syntax, Markdown is a first-level markup language for Sphinx.

The syntax for sphinx directives with MyST markdown is:

```{directivename}
:options:
content
```

There are Sphinx extensions to support in-line text-based diagram generators for:

• plantuml (directive: uml)
• graphviz (directive: graphviz)
• ditaa (directive: ditaa
• mermaid (directive: mermaid)

For example with plantuml, and a custom alias ("plantuml") for the .. uml:: directive, I would write:

# Document with plantuml Diagram

```{plantuml}
@startuml
A -> B
B -> A
@enduml
```

Now, I'd like to have this fenced code block rendered inline as PlantUML diagram in the enhanced markdown preview. Btw., this works with the VS Code standard Markdown previewer and the jebbs.plantuml extension.

I have already checked out in the code that this could be implemented by extending a regular expression in process-graphs.ts... I could open a pull request. Would you be open to include this change?