Recommend using puml as file extension

[jgarte]

Hi,

It seems like a more common file extension for plantuml files is .puml.

Or should we mention both formats?

What do you think?

https://en.wikipedia.org/wiki/PlantUML

[casch-at]

Was just about to create a PR, but thought I look into the PRs might not hurt. Anyway, currently plantuml-mode is associated with the following extensions .plantuml .pum .plu..puml seems to be the more common extension beside .plantuml, see e.g.:
https://github.com/esteinberg/plantuml4idea/blob/475d8ec04f0e29efd10975141ac7ea97da8a1973/src/main/java/org/plantuml/idea/lang/PlantUmlFileType.java#L15
qjebbs/vscode-plantuml#138 (comment)