Add DOM#id suffix spec, use dom.id() in README examples

Missing spec coverage for the suffixes parameter on DOM#id. README
examples were manually interpolating ids instead of using the API.

Author: bradgessler

README.md

@@ -412,7 +412,7 @@ class SignupForm < Components::Form
     fieldset do
       legend { "Roles" }
       Role.all.each do |role|
-        label(for: "#{field(:role_ids).dom.id}_#{role.id}") do
+        label(for: field(:role_ids).dom.id(role.id)) do
           Field(:role_ids).checkbox(value: role.id)
           whitespace
           plain role.name
@@ -426,7 +426,7 @@ class SignupForm < Components::Form
     fieldset do
       legend { "Plan" }
       Plan.all.each do |plan|
-        label(for: "#{field(:plan_id).dom.id}_#{plan.id}") do
+        label(for: field(:plan_id).dom.id(plan.id)) do
           Field(:plan_id).radio(plan.id)
           whitespace
           plain plan.name

spec/superform/dom_spec.rb

@@ -72,5 +72,11 @@ def build_lineage(**lineage)
 
       expect(field.dom.id).to eq("grandparent_bars_baz")
     end
+
+    it "appends suffixes to the id" do
+      field = build_lineage(parent: Superform::Namespace, child: Superform::Field)
+      expect(field.dom.id("male")).to eq("parent_child_male")
+      expect(field.dom.id(1)).to eq("parent_child_1")
+    end
   end
 end