Enabled frozen string literals (#1154)

* Enabled frozen string literals

* Update rubocop config

* Prefer string interpolation in simple cases

Co-Authored-By: Dylan Thacker-Smith <[email protected]>

lib/liquid/errors.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Liquid
   class Error < ::StandardError
     attr_accessor :line_number
@@ -5,7 +7,7 @@ module Liquid
     attr_accessor :markup_context
 
     def to_s(with_prefix = true)
-      str = ""
+      str = +""
       str << message_prefix if with_prefix
       str << super()
 
@@ -20,7 +22,7 @@ module Liquid
     private
 
     def message_prefix
-      str = ""
+      str = +""
       str << if is_a?(SyntaxError)
         "Liquid syntax error"
       else