Allow changes to yarn.lock files

This will allow Javascript dependencies to be automatically merged.

Example of PR that would be affected by this change: alphagov/travel-advice-publisher#1890

I have not added `Gemfile` and `package.json` to the allowed files list because in some cases we may have
legitimate reasons to keep dependencies pinned to specific versions and we don't want Dependabot to change that.

lib/pull_request.rb

@@ -38,10 +38,10 @@ def validate_single_commit
   end
 
   def validate_files_changed
+    allowed_files = ["yarn.lock", "Gemfile.lock"]
     commit = GitHubClient.instance.commit("alphagov/#{@api_response.base.repo.name}", @api_response.head.sha)
     files_changed = commit.files.map(&:filename)
-    # TODO: support other package managers too (e.g. NPM)
-    files_changed == ["Gemfile.lock"]
+    (files_changed - allowed_files).empty? && (files_changed & allowed_files).any?
   end
 
   def validate_ci_workflow_exists