Register type and file extension

[linonetwo]

Like

https://github.com/tiddly-gittly/tiddlywiki-whiteboard/blob/master/src/Startup/registerFileType.js

So it will appear in the type list as an suggestion.

[linonetwo]

And register extension so we can use .mermaid in https://tiddlywiki.com/#Typed%20Blocks%20in%20WikiText like how $$$.svg works