Constructor override is out-of-sync with Ruby 3.3.0's Logger class

[st0012]

After ruby/logger#85, Logger#initialize creates a @level_override = {} and access it every time level is called .

Since MonoLogger overrides the constructor but doesn't initialises the same @level_override variable, it now raises NoMethodError whenever level is called.

irb(main):001:0> require "mono_logger"
=> true
irb(main):002:0> MonoLogger.new(nil).level
/opt/rubies/3.3.0-dev/lib/ruby/3.3.0+0/logger.rb:385:in `level': undefined method `[]' for nil (NoMethodError)
                                                                                          
    @level_override[Fiber.current] || @level                                              
                   ^^^^^^^^^^^^^^^                                                        
        from (irb):2:in `<main>'                                                          
        from <internal:kernel>:187:in `loop'                                              

[st0012]

Should be fixed by #12

[st0012]

Closing as #12 has been merged