grammar: support spaces and doc with the `in "Java"` syntax

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge: grammar: support spaces and doc with the `in "Lang"` syntax

Enables:
~~~~
extern class A in "Java"
    `{ MyJavaClass `}

# ...
end
~~~~

Still needs work for `
` after imports...

Pull-Request: #504
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>