Add alias for plantuml directive

[stephanlachnit]

Currently, only the uml directive is supported. It would be nice if plantuml as a alias to the uml directive could be added.

The main argument for this is support for PlantUML rendering with GitLab in Markdown. It supports PlantUML via:

```plantuml
Bob -> Alice : hello
Alice -> Bob : hi
```

If you use MyST to use Markdown with sphinx, PlantUML via sphinxcontrib.plantuml only works with a {uml} code fence:

```{uml}
Bob -> Alice : hello
Alice -> Bob : hi
```

This is unfortunate because this means you can't use the same file to render both with GitLab and sphinx.

However, if the plantuml directive would be supported, one could set

myst_fence_as_directive = ["plantuml"]

in the sphinx config to support the default GitLab rendering.