Add `sse_erb_support` feature flag

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/235176
Feature flag issue: https://gitlab.com/gitlab-org/gitlab/-/issues/235460

There is uncertainty around parsing `.md.erb` files. To be on a safe
side, we want to control Static Site Editor support for ERB files.

config/feature_flags/development/sse_erb_support.yml

+---
+name: sse_erb_support
+introduced_by_url:
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/235460
+group: group::static site editor
+type: development
+default_enabled: false

lib/gitlab/static_site_editor/config.rb

@@ -3,7 +3,7 @@
 module Gitlab
   module StaticSiteEditor
     class Config
-      SUPPORTED_EXTENSIONS = %w[.md .md.erb].freeze
+      SUPPORTED_EXTENSIONS = %w[.md].freeze
 
       def initialize(repository, ref, file_path, return_url)
         @repository = repository
@@ -42,6 +42,8 @@ module Gitlab
       end
 
       def extension_supported?
+        return true if file_path.end_with?('.md.erb') && Feature.enabled?(:sse_erb_support, project)
+
         SUPPORTED_EXTENSIONS.any? { |ext| file_path.end_with?(ext) }
       end
 

spec/lib/gitlab/static_site_editor/config_spec.rb

@@ -46,8 +46,6 @@ RSpec.describe Gitlab::StaticSiteEditor::Config do
     end
 
     context 'when file has .md.erb extension' do
-      let(:file_path) { 'README.md.erb' }
-
       before do
         repository.create_file(
           project.creator,
@@ -58,7 +56,25 @@ RSpec.describe Gitlab::StaticSiteEditor::Config do
         )
       end
 
-      it { is_expected.to include(is_supported_content: 'true') }
+      context 'when feature flag is enabled' do
+        let(:file_path) { 'FEATURE_ON.md.erb' }
+
+        before do
+          stub_feature_flags(sse_erb_support: project)
+        end
+
+        it { is_expected.to include(is_supported_content: 'true') }
+      end
+
+      context 'when feature flag is disabled' do
+        let(:file_path) { 'FEATURE_OFF.md.erb' }
+
+        before do
+          stub_feature_flags(sse_erb_support: false)
+        end
+
+        it { is_expected.to include(is_supported_content: 'false') }
+      end
     end
 
     context 'when file path is nested' do