Allow .mermaid as file extension

[henrikrxn]

According to the Mermaid documentation .mermaid is also a file extension for mermaid files

https://mermaid.js.org/ecosystem/integrations.html#file-extension

[bpruitt-goddard]

This should already be working as a file extension. The files in the tests directory are all using the .mermaid extension and are properly highlighting.