Fall back on "placeholder" when "placeholder-text" is not used

While "placeholder-text" usage predates aria-placeholder, some user agent
implementors would prefer to expose placeholder text using "placeholder"
because doing so is consistent with the convention used for other ARIA
properties. So check for both.

src/orca/generator.py

@@ -369,6 +369,10 @@ class Generator:
         if placeholder:
             return [placeholder]
 
+        placeholder = attrs.get('placeholder')
+        if placeholder:
+            return [placeholder]
+
         return []
 
     def _generateLabelAndName(self, obj, **args):