Add frozen_string_literal comments

This ensures we're set up for the eventual migration to Ruby 3, which
will enforce immutable strings by default. We keep the explicit calls to
`#freeze` since Ruby versions older than 2.3 won't respect this magic
comment.

Author: sds

lib/overcommit/constants.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Global application constants.
 module Overcommit
   HOME = File.expand_path(File.join(File.dirname(__FILE__), '..', '..')).freeze

lib/overcommit/hook/pre_commit/berksfile_check.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Overcommit::Hook::PreCommit
   # Check if local Berksfile.lock matches Berksfile when either changes, unless
   # Berksfile.lock is ignored by git.

lib/overcommit/hook/pre_commit/bundle_check.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Overcommit::Hook::PreCommit
   # Check if local Gemfile.lock matches Gemfile when either changes, unless
   # Gemfile.lock is ignored by git.

lib/overcommit/message_processor.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Overcommit
   # Utility class that encapsulates the handling of hook messages and whether
   # they affect lines the user has modified or not.

lib/overcommit/version.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Defines the gem version.
 module Overcommit
   VERSION = '0.30.0'.freeze