Add accepts_block? macromethod to Def

[willhbr]

Works around #5334 by adding .accepts_block? macro method which is true if the method can be called with a block. This is useful for macros that want to dispatch correctly to a method.

The example code from that bug (edited to use the new method):

class Foo
  macro finished
    {% for method in @type.methods %}
      {% puts method.accepts_block? %}
    {% end %}
  end

  def no_block
  end

  def just_yield
    yield 4
  end

  def yield_and_block(&block)
    yield 4
  end

  def yield_and_block_with_type(&block : Int32 ->)
    yield 4
  end
end

Gives the output as expected:

false
true
true
true

[bcardiff]

Adding this to 0.26.1 since it would make it possible to use it in the stdlib for 0.27.0