DEV: Allow fab! without block (#24314)

The most common thing that we do with fab! is:

    fab!(:thing) { Fabricate(:thing) }

This commit adds a shorthand for this which is just simply:

    fab!(:thing)

i.e. If you omit the block, then, by default, you'll get a `Fabricate`d object using the fabricator of the same name.

spec/integration/watched_words_spec.rb

@@ -2,10 +2,10 @@
 
 RSpec.describe WatchedWord do
   fab!(:tl2_user) { Fabricate(:user, trust_level: TrustLevel[2]) }
-  fab!(:admin) { Fabricate(:admin) }
-  fab!(:moderator) { Fabricate(:moderator) }
+  fab!(:admin)
+  fab!(:moderator)
 
-  fab!(:topic) { Fabricate(:topic) }
+  fab!(:topic)
   fab!(:first_post) { Fabricate(:post, topic: topic) }
 
   let(:require_approval_word) do