[ruby/irb] process multi-line pastes as a single entity

this allows pasting leading-dot chained methods correctly:

```ruby
class A
  def a; self; end
  def b; true; end
end

a = A.new

a
 .a
 .b
```

will properly return `true` instead of erroring on the `.a` line:

```
irb(main):001:1*     class A
irb(main):002:1*       def a; self; end
irb(main):003:0>     end
irb(main):004:0*
irb(main):005:0>     a = A.new
irb(main):006:0*
irb(main):007:0>     a
irb(main):008:0>      .a
irb(main):009:0>      .a
=> #<A:0x00007f984211fbe8>
```

https://github.com/ruby/irb/commit/45aeb52575

lib/irb/input-method.rb

@@ -316,7 +316,13 @@ module IRB
       Reline.output = @stdout
       Reline.prompt_proc = @prompt_proc
       Reline.auto_indent_proc = @auto_indent_proc if @auto_indent_proc
-      if l = readmultiline(@prompt, false, &@check_termination_proc)
+
+      l = readmultiline(@prompt, false) do |line|
+        next false if Reline::IOGate.in_pasting?
+        @check_termination_proc.call(line)
+      end
+
+      if l
         HISTORY.push(l) if !l.empty?
         @line[@line_no += 1] = l + "\n"
       else