Syntax checking allows for variables in Turtle

[RinkeHoekstra]

This is probably because the syntax definition is based on the SPARQL syntax that YASQE/YASR use, but it can be confusing to novices in the field.

Shouldn't be too hard to disallow the variables for appearing.

[jiemakel]

The syntax definition is actually a rewrite of this one from CodeMirror. It is unfortunately very lax in what it accepts (for example, the present problem arises because it doesn't actually require a prefix in a prefixed name, and in addition thinks these can start with a ?).

A proper fix would be to avail of the much better lexer from N3.js. However, some sort of wrapper would have to be built that would allow the lexing to continue after any encountered errors (which is important to the error highlighting scenario here, but not covered by the parsing-oriented N3.js lexer).

This has been an issue that I've known about for long, but not had time to investigate (and will likely not have in the near future). Pull requests naturally encouraged.