Replace usage of cgi for ruby 3.5 compatibility (#2131)

In Ruby 3.5, most of the cgi gem is removed. That includes `CGI.parse`, which `rouge` uses.

But, we can subsistute it with `URI.decode_www_form`.
The data is returned in a slightly different format (array of arrays instead of a hash),
so do some processing to get back into the expected structure.

https://bugs.ruby-lang.org/issues/21258

Author: Earlopain

lib/rouge/cli.rb

@@ -5,6 +5,7 @@
 # to use this module, require 'rouge/cli'.
 
 require 'rbconfig'
+require 'uri'
 
 module Rouge
   class FileReader
@@ -348,7 +349,7 @@ def run
       end
 
     private_class_method def self.parse_cgi(str)
-        pairs = CGI.parse(str).map { |k, v| [k.to_sym, v.first] }
+        pairs = URI.decode_www_form(str).map { |k, v| [k.to_sym, v] }
         Hash[pairs]
       end
     end

lib/rouge/formatters/html_legacy.rb

@@ -1,9 +1,6 @@
 # -*- coding: utf-8 -*- #
 # frozen_string_literal: true
 
-# stdlib
-require 'cgi'
-
 module Rouge
   module Formatters
     # Transforms a token stream into HTML output.

lib/rouge/lexer.rb

@@ -3,8 +3,8 @@
 
 # stdlib
 require 'strscan'
-require 'cgi'
 require 'set'
+require 'uri'
 
 module Rouge
   # @abstract
@@ -52,17 +52,19 @@ def lookup_fancy(str, code=nil, default_options={})
         name, opts = str ? str.split('?', 2) : [nil, '']
 
         # parse the options hash from a cgi-style string
-        opts = CGI.parse(opts || '').map do |k, vals|
-          val = case vals.size
+        cgi_opts = Hash.new { |hash, key| hash[key] = [] }
+        URI.decode_www_form(opts || '').each do |k, val|
+          cgi_opts[k] << val
+        end
+        cgi_opts.transform_values! do |vals|
+          case vals.size
           when 0 then true
           when 1 then vals[0]
           else vals
           end
-
-          [ k.to_s, val ]
         end
 
-        opts = default_options.merge(Hash[opts])
+        opts = default_options.merge(cgi_opts)
 
         lexer_class = case name
         when 'guess', nil