Support eval "return" at toplevel

Since Ruby 2.4, `return` is supported at toplevel.  This makes `eval "return"`
also supported at toplevel.

This mostly uses the same tests as direct `return` at toplevel, with a couple
differences:

`END {return if false}` is a SyntaxError, but `END {eval "return" if false}`
is not an error since the eval is never executed. `END {return}` is a
SyntaxError, but `END {eval "return"}` is a LocalJumpError.

The following is a SyntaxError:

```ruby
class X
nil&defined?0--begin e=no_method_error(); return; 0;end
end
```

However, the following is not, because the eval is never executed:

```ruby
class X
nil&defined?0--begin e=no_method_error(); eval "return"; 0;end
end
```

Fixes [Bug #19779]

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