Allow overriding admin component templates

Unfortunately, view_component doesn't allow overriding templates from
the host application by just having the same relative path [1].

For now, we'll use the inline templates feature from v3 [2] so that host
applications can override only the template while still inheriting the
rest of view logic.

[1] - ViewComponent/view_component#411
[2] - https://viewcomponent.org/guide/templates.html#inline

admin/app/components/solidus_admin/base_component.rb

@@ -3,6 +3,8 @@
 module SolidusAdmin
   # BaseComponent is the base class for all components in Solidus Admin.
   class BaseComponent < ViewComponent::Base
+    include ViewComponent::InlineTemplate
+
     def container
       SolidusAdmin::Container
     end

admin/app/components/solidus_admin/main_nav_component.rb

@@ -3,5 +3,14 @@
 module SolidusAdmin
   # Renders the main navigation of Solidus Admin.
   class MainNavComponent < BaseComponent
+    erb_template <<~ERB
+      <nav>
+        <%=
+          render component("main_nav_item").with_collection(
+            container.all("main_nav").sort_by(&:position)
+          )
+        %>
+      </nav>
+    ERB
   end
 end

admin/app/components/solidus_admin/main_nav_item_component.rb

@@ -11,5 +11,11 @@ def initialize(item:)
       @item = item
       super
     end
+
+    erb_template <<~ERB
+      <a href="#">
+        <%= item.title %>
+      </a>
+    ERB
   end
 end