fix: make alt text mandatory on image

Signed-off-by: Filipe Medeiros <hello@filipesm.eu>

studio/schemas/blogPost.ts

@@ -83,6 +83,12 @@ const blogPost = defineType({
               type: 'array',
               of: [{type: 'block'}],
             },
+            {
+              title: 'Alt text',
+              name: 'altText',
+              type: 'string',
+              validation: (r) => r.required(),
+            },
           ],
         },
       ],