FEATURE: modifier API for plugins (#20887)

Introduces a new API for plugin data modification without class-based extension overhead.

This commit introduces a new API that allows plugins to modify data in cases where they return different data rather than additional data, as is common with filtered_registers in DiscoursePluginRegistry. This API removes the need for defining class-based extension points.

When a plugin registers a modifier, it will automatically be called if the plugin is enabled. The core will then modify the parameter sent to it using the block registered by the plugin:
 
```ruby
DiscoursePluginRegistry.register_modifier(plugin_instance, :magic_sum_modifier) { |a, b| a + b }
sum = DiscoursePluginRegistry.apply_modifier(:magic_sum_filter, 1, 2)
expect(sum).to eq(3)
```

Key features of these modifiers:

- Operate in a stack (first registered, first called)
- Automatically disabled when the plugin is disabled
- Pass the cumulative result of all block invocations to the caller

spec/lib/plugin/instance_spec.rb

@@ -827,4 +827,17 @@ RSpec.describe Plugin::Instance do
       expect(callback_called).to eq(false)
     end
   end
+
+  describe "#register_modifier" do
+    let(:plugin) { Plugin::Instance.new }
+
+    after { DiscoursePluginRegistry.clear_modifiers! }
+
+    it "allows modifier registration" do
+      plugin.register_modifier(:magic_sum_modifier) { |a, b| a + b }
+
+      sum = DiscoursePluginRegistry.apply_modifier(:magic_sum_modifier, 1, 2)
+      expect(sum).to eq(3)
+    end
+  end
 end