Replace CGI with URI for Ruby 3.5 support

In Ruby 3.5, most of the cgi gem is removed which includes `CGI.parse`. We can replace this with `URI.decode_www_form` but it has some slight differences. Seems any blank value now always comes back as nil rather nil or an empty string depending so this can be a breaking change.

rouge-ruby/rouge#2131

Author: javierjulio

lib/envied/coercer/envied_string.rb

@@ -27,8 +27,8 @@ def to_float(str)
   end
 
   def to_hash(str)
-    require 'cgi'
-    ::CGI.parse(str).map { |key, values| [key, values[0]] }.to_h
+    require 'uri'
+    ::URI.decode_www_form(str).map { |key, values| [key, values[0]] }.to_h
   end
 
   def to_string(str)

spec/coercer_spec.rb

@@ -255,7 +255,7 @@ def coerce(str, type)
         {
           'a=1' => {'a' => '1'},
           'a=1&b=2' => {'a' => '1', 'b' => '2'},
-          'a=&b=2' => {'a' => '', 'b' => '2'},
+          'a=&b=2' => {'a' => nil, 'b' => '2'},
           'a&b=2' => {'a' => nil, 'b' => '2'},
         }.each do |value, hash|
           expect(coerce(value, :hash)).to eq hash

spec/envied_spec.rb

@@ -282,7 +282,7 @@ def envied_require(*args, **options)
         end
 
         it 'yields hash from string' do
-          expect(ENVied.FOO).to eq({ 'a' => '1', 'b' => '', 'c' => nil })
+          expect(ENVied.FOO).to eq({ 'a' => '1', 'b' => nil, 'c' => nil })
         end
 
         it 'yields hash from an empty string' do