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/wizard/wizard_builder_spec.rb

@@ -99,7 +99,7 @@ RSpec.describe Wizard::Builder do
   describe "styling" do
     let(:styling_step) { wizard.steps.find { |s| s.id == "styling" } }
     let(:font_field) { styling_step.fields[1] }
-    fab!(:theme) { Fabricate(:theme) }
+    fab!(:theme)
     let(:colors_field) { styling_step.fields.first }
 
     it "has the full list of available fonts" do