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.

plugins/discourse-local-dates/spec/system/local_dates_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 describe "Local dates", type: :system do
-  fab!(:topic) { Fabricate(:topic) }
+  fab!(:topic)
   fab!(:current_user) { Fabricate(:user) }
   let(:year) { Time.zone.now.year + 1 }
   let(:month) { Time.zone.now.month }