grammar: allow '`' inside foreign code blocks

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge: grammar: allow '`' inside foreign code blocks

This was an old bug causing the use of ``` ` ``` within a foreign code block to break the block. Example:

~~~~
fun foo `{
    // Some comment on entity `foo` would prevent the whole block to be recognize
`}
~~~~

Pull-Request: #847
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>