[Feature]: Relative tinker path

[Roboroads]

As pointed out in #249, there is one additional case you could implement.

`$app = require_once DIR . $relativeCustomProjectDirectory . '/bootstrap/app.php';

But for this, you require an option that denotes that path of the plugin path input has to be treated as relative.

Originally posted by @dennyweiss in #250 (comment)

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