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/lib/pretty_text/helpers_spec.rb

@@ -26,7 +26,7 @@ RSpec.describe PrettyText::Helpers do
         description: "Really great stuff here",
       )
     end
-    fab!(:user) { Fabricate(:user) }
+    fab!(:user)
 
     it "handles tags and categories based on slug with type suffix" do
       expect(