Add lint to default rake tasks

Currently when we run bundle exec rake only rspec runs. Adding a lint rake task, and including this with the default rake tasks to match search-api and search-api-v2-beta-features

Author: stringiest

Rakefile

@@ -5,4 +5,6 @@ require_relative "config/application"
 
 Rails.application.load_tasks
 
-task default: %i[spec]
+# Undo any existing default tasks added by dependencies so we can redefine the task
+Rake::Task[:default].clear if Rake::Task.task_defined?(:default)
+task default: %i[lint spec]

lib/tasks/lint.rake

@@ -0,0 +1,4 @@
+desc "Run all linters"
+task lint: :environment do
+  sh "bundle exec rubocop"
+end