Interpreter: allow inspecting block vars without affecting program

[asterite]

Fixes #12518

This would would also be good for 1.6

[asterite]

I also fixed another bug that I noticed that happened when you introduced new vars while debugging inside a block.

So now the only thing remaining is being able to define new vars inside a block and later change their type (similar to what you can do at the top level,) but I'll leave that for another PR.

[asterite]

Here's an example session showing how it works now:

✗ bin/crystal i foo.cr
Using compiled compiler at .build/crystal
From: foo.cr:6:44 InsertionSort.class#sort:

     1: class InsertionSort
     2:   def self.sort(arr)
     3:     (1..arr.size - 1).each do |i|
     4:       (0..i - 1).each do |j|
     5:         debugger
 =>  6:         arr[j], arr[i] = arr[i], arr[j] if arr[i] < arr[j]
     7:       end
     8:     end
     9:   end
    10: end
    11:

pry(InsertionSort.sort)> i
=> 1
pry(InsertionSort.sort)> j
=> 0
pry(InsertionSort.sort)> arr
=> [4, 8, 7, 2, 6, 5, 3, 0, 9, 1]
pry(InsertionSort.sort)> b = [1, 2, 3]
=> [1, 2, 3]
pry(InsertionSort.sort)> b
=> [1, 2, 3]
pry(InsertionSort.sort)> b.reverse
=> [3, 2, 1]
pry(InsertionSort.sort)> continue
From: foo.cr:6:44 InsertionSort.class#sort:

     1: class InsertionSort
     2:   def self.sort(arr)
     3:     (1..arr.size - 1).each do |i|
     4:       (0..i - 1).each do |j|
     5:         debugger
 =>  6:         arr[j], arr[i] = arr[i], arr[j] if arr[i] < arr[j]
     7:       end
     8:     end
     9:   end
    10: end
    11:

pry(InsertionSort.sort)> i
=> 2
pry(InsertionSort.sort)> j
=> 0
pry(InsertionSort.sort)> continue
From: foo.cr:6:44 InsertionSort.class#sort:

     1: class InsertionSort
     2:   def self.sort(arr)
     3:     (1..arr.size - 1).each do |i|
     4:       (0..i - 1).each do |j|
     5:         debugger
 =>  6:         arr[j], arr[i] = arr[i], arr[j] if arr[i] < arr[j]
     7:       end
     8:     end
     9:   end
    10: end
    11:

pry(InsertionSort.sort)> ^C

[zw963]

Add to milestone 1.6 ?