Support for frozen string literals

When run with the --frozen_string_literal flag, Ruby will throw an error
if you try to modify a string object.

Leaving discussions of whether this approach is idiomatic Ruby (or not)
to one side, it seems sensible in light of the discussion in [1] and [2]
for projects like Nesta to work without mutating strings. And note that
in Ruby 3.4, things are changing.

To find the areas of the code that warranted an update I ran the tests
under Ruby 3.3, like this:

    RUBYOPT=--enable=frozen-string-literal rake test

[1] https://bugs.ruby-lang.org/issues/16047
[2] https://bugs.ruby-lang.org/issues/20205

lib/nesta/commands/plugin/create.rb

@@ -35,11 +35,11 @@ def nested_module_definition_with_version
             indent_levels << indent_level
           end
 
-          ''.tap do |code|
-            lines.each_with_index do |line, i|
-              code << '  ' * (indent_levels[i] + 2) + line
-            end
+          code = []
+          lines.each_with_index do |line, i|
+            code << '  ' * (indent_levels[i] + 2) + line
           end
+          code.join("\n")
         end
 
         def make_directories

lib/nesta/config_file.rb

@@ -10,7 +10,7 @@ def set_value(key, value)
 
       configured = false
       File.open(self.class.path, 'r+') do |file|
-        output = ''
+        output = []
         file.each_line do |line|
           if configured
             output << line
@@ -21,7 +21,7 @@ def set_value(key, value)
         end
         output << "#{replacement}\n" unless configured
         file.pos = 0
-        file.print(output)
+        file.print(output.join("\n"))
         file.truncate(file.pos)
       end
     end

lib/nesta/helpers.rb

@@ -90,7 +90,7 @@ def articles_heading
       #   path_to(page.abspath, uri: true)
       #
       def path_to(page_path, options = {})
-        host = ''
+        host = []
         if options[:uri]
           host << "http#{'s' if request.ssl?}://"
           if (request.env.include?("HTTP_X_FORWARDED_HOST") or 
@@ -100,7 +100,7 @@ def path_to(page_path, options = {})
             host << request.host
           end
         end
-        uri_parts = [host]
+        uri_parts = [host.join('')]
         uri_parts << request.script_name.to_s if request.script_name
         uri_parts << page_path
         File.join(uri_parts)

lib/nesta/models/menu.rb

@@ -16,7 +16,7 @@ def self.top_level
     end
 
     def self.for_path(path)
-      path.sub!(Regexp.new('^/'), '')
+      path = path.sub(Regexp.new('^/'), '')
       if path.empty?
         full_menu
       else