`RubyVM.keep_script_lines` enables to keep script lines
for each ISeq and AST. This feature is for debugger/REPL
support.

```ruby
RubyVM.keep_script_lines = true
RubyVM::keep_script_lines = true

eval("def foo = nil\ndef bar = nil")
pp RubyVM::InstructionSequence.of(method(:foo)).script_lines
```