Fix colorize backtick symbol (#508)

Co-authored-by: Stan Lo <stan001212@gmail.com>

Author: tompng

lib/irb/color.rb

@@ -242,7 +242,7 @@ def scan_token(token)
         case token
         when :on_symbeg, :on_symbols_beg, :on_qsymbols_beg
           @stack << true
-        when :on_ident, :on_op, :on_const, :on_ivar, :on_cvar, :on_gvar, :on_kw
+        when :on_ident, :on_op, :on_const, :on_ivar, :on_cvar, :on_gvar, :on_kw, :on_backtick
           if @stack.last # Pop only when it's Symbol
             @stack.pop
             return prev_state

test/irb/test_color.rb

@@ -90,6 +90,7 @@ def test_colorize_code
         ":class" => "#{YELLOW}:#{CLEAR}#{YELLOW}class#{CLEAR}",
         "[:end, 2]" => "[#{YELLOW}:#{CLEAR}#{YELLOW}end#{CLEAR}, #{BLUE}#{BOLD}2#{CLEAR}]",
         "[:>, 3]" => "[#{YELLOW}:#{CLEAR}#{YELLOW}>#{CLEAR}, #{BLUE}#{BOLD}3#{CLEAR}]",
+        "[:`, 4]" => "[#{YELLOW}:#{CLEAR}#{YELLOW}`#{CLEAR}, #{BLUE}#{BOLD}4#{CLEAR}]",
         ":Hello ? world : nil" => "#{YELLOW}:#{CLEAR}#{YELLOW}Hello#{CLEAR} ? world : #{CYAN}#{BOLD}nil#{CLEAR}",
         'raise "foo#{bar}baz"' => "raise #{RED}#{BOLD}\"#{CLEAR}#{RED}foo#{CLEAR}#{RED}\#{#{CLEAR}bar#{RED}}#{CLEAR}#{RED}baz#{CLEAR}#{RED}#{BOLD}\"#{CLEAR}",
         '["#{obj.inspect}"]' => "[#{RED}#{BOLD}\"#{CLEAR}#{RED}\#{#{CLEAR}obj.inspect#{RED}}#{CLEAR}#{RED}#{BOLD}\"#{CLEAR}]",