Merge: grammar: add union and intersection types

This is a stub in the grammar to add intersection and union types.

* `var x: A and B`
* `var x: A or B and C or D or E`

Adding parenthesis `(A and B) or C` cause some nasty conflicts because the Nit grammar is very complex and organic (and sablecc is nowadays more a curse than a blessing).

all usage of types accept intersection and union except `new` and `isa` because LR1 conflicts with the boolean `or`.

* `x isa A or...`: shift type (`x isa A or B`) vs. reduce expr (`x isa A or true`)

This is not too much a problem because:

* intersection and union type are abstract (no `new`)
* `a isa A or B` should be the same than `a isa A or a isa B`

Note that this is only a stub: only the grammar is extended:
* there is no new AST node (`A or B` is parsed as `A`, the `B` is just dropped)
* there is no change in the metamodel of Nit

Pull-Request: #2389
Reviewed-by: Jean-Christophe Beaupré <jcbrinfo.public@gmail.com>