MRI: rewrite script eval (was: Question about $0)

Maybe there is a reason for setting `$0`. Could you tell me? I can only speculate:

It's far easier and more intuitive than this which doesn't work with mkxp:
```ruby
$RGSS_SCRIPTS[__FILE__[/Section(\d+)/, 1].to_i][0]  # RGSS1 (and 2?)
```

So if that's a mkxp-specific extension, then it should be mentioned in the README.