Check affix flags

This is an oversight from porting spylls' `is_good_form` method. We need
to check that a prefix is allowed to be used on a stem by comparing the
Word's flag set against the affix's flag. This fixes lookups for
incorrect words like "undrink". It's a valid construction of a prefix
("un") with a word from the dic ("drink") but the "un" prefix is not
in "drink"'s flags, so it should be rejected.

src/checker.rs

@@ -494,6 +494,17 @@ impl<'a> Checker<'a> {
             }
         }
 
+        if is_some_and(form.prefixes[0].as_ref(), |prefix| {
+            !all_flags.contains(&prefix.flag)
+        }) {
+            return false;
+        }
+        if is_some_and(form.suffixes[0].as_ref(), |suffix| {
+            !all_flags.contains(&suffix.flag)
+        }) {
+            return false;
+        }
+
         if let Some(flag) = self.aff.circumfix_flag {
             let has_suffix = is_some_and(form.suffixes[0].as_ref(), |suffix| {
                 suffix.flags.contains(&flag)