Allow inspecting implicit parameters via `implicit_parameter_get` etc.

Currently, it is not possible to know what implicit parameters like it/_1 actually are. If I want to know, I have to rewrite the block to take explicit parameters.

[1, 2, 3].each do
  puts it
  binding.irb
end

$ ruby test.rb
1

From: test.rb @ line 3 :

    1: [1, 2, 3].each do
    2:   puts it
 => 3:   binding.irb
    4: end

irb(main):001> it
/home/user/code/test/test.rb(irb):1:in 'block in <main>': undefined local variable or method 'it' for main (NameError)
        from test.rb:3:in 'Binding#irb'
        from test.rb:3:in 'block in <main>'
        from test.rb:1:in 'Array#each'
        from test.rb:1:in '<main>'

Ruby 4.0 actually added methods to interogate the values of such params (https://bugs.ruby-lang.org/issues/21049#note-7) and it would be great if it can just work in irb.

Start by reading Ruby 4.0's implicit-parameter interrogation methods referenced in the linked Ruby issue and the binding.irb behavior shown in the example. Trace how IRB evaluates names such as it and _1, then verify that implicit_parameter_get and related methods can inspect them from the session without rewriting the block.