[ruby-core:70982] limiting scope of magic comments like frozen_string_literal

How about being able to limit the scope of magic comments like
how the "use" / "no" pragmas work in Perl?

So something like:

	def hello_world
	  # frozen_string_literal: true
	  puts 'Hello world'

	  # frozen_string_literal: false
	  foo = 'Hello'
	  foo << ' world'
	end

It would make refactoring to moving methods to different files/modules
easier, I think.