Merge: gtksourceview: Rewrite the language definition

The new definition includes:
* A more accurate and up-to-date grammar
* A more accurate style list
* A list of built-in annotations
* Heuristics for embedded languages
* Spell-checking only enabled for comments
* Styles for operators and lower-case identifiers available if needed
  (but not highlighted by default)

## Previews
Here are the PDF outputs of various black-on-white style schemes for `examples/syntax.nit`:
* [Classic](https://github.com/nitlang/nit/files/1109793/gtksourceview-classic.pdf)
* [Kate](https://github.com/nitlang/nit/files/1109794/gtksourceview-kate.pdf)
* [Solarized Clear](https://github.com/nitlang/nit/files/1109795/gtksourceview-solarized-clear.pdf)
* [Tango](https://github.com/nitlang/nit/files/1109796/gtksourceview-tango.pdf)

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>

Pull-Request: #2515
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>