Add prefix validation / highlight when a prefix is not declared in the header

[tfrancart]

Having the ability to highlight prefixes in a Turtle or SPARQL file that are not declared in the header of the file would be great !
Thanks for making this package available !

[blake-regalia]

I have thought about this as well, it would actually require a linter plugin. I'll keep this issue open and add it to my todo list :)