Add frozen string literal comment to files.

Add frozen string literal comment to files.

diff --git a/plugin.rb b/plugin.rb
index f3cdb5a..0c8e8a0 100644
--- a/plugin.rb
+++ b/plugin.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # name: discourse-spoiler-alert
 # about: Uses the Spoiler Alert plugin to blur text when spoiling it.
 # version: 0.4
diff --git a/spec/post_alerter_spec.rb b/spec/post_alerter_spec.rb
index faaa963..2ae7f21 100644
--- a/spec/post_alerter_spec.rb
+++ b/spec/post_alerter_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'rails_helper'
 
 RSpec.describe PostAlerter do
diff --git a/spec/pretty_text_spec.rb b/spec/pretty_text_spec.rb
index 7f7b74b..8b4c297 100644
--- a/spec/pretty_text_spec.rb
+++ b/spec/pretty_text_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'rails_helper'
 
 describe PrettyText do

GitHub sha: f100ee2e