grammar: add syntax for typed literal arrays

~~~
var a = [1, 2, 1.5: Numeric]
~~~

Signed-off-by: Jean Privat <jean@pryen.org>
parser: regenerate for typed literal arrays

Signed-off-by: Jean Privat <jean@pryen.org>
typing: adapt for typed literal arrays.

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add base_array_lit_typed.nit

Signed-off-by: Jean Privat <jean@pryen.org>