Make argparse reject supplying a validator for boolean flags

Specifically, this commit simply makes argparse issue an error if you use the !
syntax to define a validation script on an option that does not take any
arguments. For example, "argparse foo!exit -- --foo" is now an error. This was
previously accepted, despite that fact that the code after ! would never be
executed (the ! code is only executed when an option is given a value).

Alternatively, ! validation scripts could be made to execute even when no value
was provided, but this break existing code that uses them with flags that take
optional values.

tests/checks/argparse.fish

@@ -678,6 +678,11 @@ begin
     #CHECKERR: (Type 'help argparse' for related documentation)
 end
 
+begin
+    argparse f!echo hello -- -f
+    #CHECKERR: argparse: Invalid option spec 'f!echo' at char '!'
+end
+
 begin
     argparse --ignore-unknown h i -- -hoa -oia
     echo -- $argv