Add support for the Laravel asset() helpers

[jaxwilko]

This PR adds some of the Laravel defaults into Winter's configs.

app.asset_url was added with reference to this PR which adds support for Laravel asset urls.

IMHO we should consider env() calls for every config that Laravel uses them for, which will be possible once #404 is merged. @bennothommo @LukeTowers thoughts?

[bennothommo]

IMHO we should consider env() calls for every config that Laravel uses them for

I cannot "yes" this enough.

[LukeTowers]

IMHO we should consider env() calls for every config that Laravel uses them for

I agree with that and once #404 is merged then the main blocker to that in the past will be able to be dealt with and we can have a configuration system that works well for newbies and pros. I'm excited for it :)