Add a benchmark of irb boot time

```
$ benchmark-driver benchmark/irb_exec.yml --rbenv '2.6.3;2.7.0-preview1'
Calculating -------------------------------------
                          2.6.3  2.7.0-preview1
            irb_exec     11.844           5.171 i/s -      30.000 times in 2.532887s 5.801960s

Comparison:
                         irb_exec
               2.6.3:        11.8 i/s
      2.7.0-preview1:         5.2 i/s - 2.29x  slower
```

benchmark/irb_exec.yml

@@ -0,0 +1,10 @@
+prelude: |
+  # frozen_string_literal: true
+  require 'rbconfig'
+  irb = File.join(File.dirname(RbConfig.ruby), 'irb')
+benchmark:
+  irb_exec: |
+    IO.popen(irb, 'w') do |io|
+      io.write('exit')
+    end
+loop_count: 30