[ruby/irb] Make IRB::ColorPrinter.pp compatible with PP.pp

The incompatible interface is not helpful, again if you want to use it
as a standalone library, falling it back to PP.

Original PP.pp also ends with `out << "\n"`.

https://github.com/ruby/irb/commit/4c74c7d84c

lib/irb/color_printer.rb

@@ -8,7 +8,7 @@ module IRB
       q = ColorPrinter.new(out, width)
       q.guard_inspect_key {q.pp obj}
       q.flush
-      out
+      out << "\n"
     end
 
     def text(str, width = nil)

lib/irb/inspector.rb

@@ -116,7 +116,7 @@ module IRB # :nodoc:
   }
   Inspector.def_inspector([true, :pp, :pretty_inspect], proc{require "irb/color_printer"}){|v|
     if IRB.conf[:MAIN_CONTEXT]&.use_colorize?
-      IRB::ColorPrinter.pp(v, '')
+      IRB::ColorPrinter.pp(v, '').chomp
     else
       v.pretty_inspect.chomp
     end

test/irb/test_color.rb

@@ -158,9 +158,9 @@ module TestIRB
         skip 'Ripper::Lexer#scan is supported in Ruby 2.7+'
       end
       {
-        1 => "#{BLUE}#{BOLD}1#{CLEAR}",
-        Struct.new('IRBTestColorPrinter', :a).new('test') => "#{GREEN}#<struct Struct::IRBTestColorPrinter#{CLEAR} a#{GREEN}=#{CLEAR}#{RED}#{BOLD}\"#{CLEAR}#{RED}test#{CLEAR}#{RED}#{BOLD}\"#{CLEAR}#{GREEN}>#{CLEAR}",
-        Ripper::Lexer.new('1').scan => "[#{GREEN}#<Ripper::Lexer::Elem:#{CLEAR} on_int@1:0 END token: #{RED}#{BOLD}\"#{CLEAR}#{RED}1#{CLEAR}#{RED}#{BOLD}\"#{CLEAR}#{GREEN}>#{CLEAR}]",
+        1 => "#{BLUE}#{BOLD}1#{CLEAR}\n",
+        Struct.new('IRBTestColorPrinter', :a).new('test') => "#{GREEN}#<struct Struct::IRBTestColorPrinter#{CLEAR} a#{GREEN}=#{CLEAR}#{RED}#{BOLD}\"#{CLEAR}#{RED}test#{CLEAR}#{RED}#{BOLD}\"#{CLEAR}#{GREEN}>#{CLEAR}\n",
+        Ripper::Lexer.new('1').scan => "[#{GREEN}#<Ripper::Lexer::Elem:#{CLEAR} on_int@1:0 END token: #{RED}#{BOLD}\"#{CLEAR}#{RED}1#{CLEAR}#{RED}#{BOLD}\"#{CLEAR}#{GREEN}>#{CLEAR}]\n",
       }.each do |object, result|
         actual = with_term { IRB::ColorPrinter.pp(object, '') }
         assert_equal(result, actual, "Case: IRB::ColorPrinter.pp(#{object.inspect}, '')")