Let @scopedslot directive be used sans parenthesis

[logan-christensen]

I often use the @scopedslot directive while passing only a single variable:

@scopedslot('name', ($user))
...
@endscopedslot

This PR would make the parenthesis syntax around the function arguments optional.

@scopedslot('name', $user)
...
@endscopedslot

My reasoning is as follows:

1. I would prefer not to type parenthesis every time
2. My code formatter (vscode-blade-formatter) will remove all parenthesis around a single variable and it causes errors.