Cast boolean env variables

[SeyamMs]

Inspired by #164

Environment variables are always casted to strings, so added PHP filter_var() to cast $_SERVER variables at if statements.

I think for argument PANTHER_DEVTOOLS its required to be able to set it to false, while for arguments PANTHER_NO_HEADLESS and PANTHER_NO_SANDBOX are false by default and true if they just defined.

Hence, another applicable solution is to convert PANTHER_DEVTOOLS to PANTHER_NO_DEVTOOLS