syntax: allow untyped variable declaration

syntax: allow untyped variable declaration

It means there is now a distinction between variables with a null static
type because of a previous error and untyped variables that also have null
static type. So introduce Variable::is_typed.

Enhance check_is_set with untyped variable verification.

Compile untyped variable as 'nullable Object' registers.

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