Refactor accessing of default slot from as var to kwarg

[JuroOravec]

See #495 (comment)

[JuroOravec]

Btw, @EmilStenstrom, what do you think of dropping the .default property when accessing the default slot?

Currently, we assign the slot to a variable with {% fill "xxx" as "alias_name" %}, and then we have to access the default slot via the "default" property as alias_name.default.

Since we already have slot_data directly on the fill tag, which we access as {% fill "xxx" slot_data="abc" %}, then IMO we should also access the default slot directly.

So instead of:

{% fill "xxx" default="orig_slot" %}
	{{ orig_slot.default }}
{% endfill %}

We'd drop the .default, and have only:

{% fill "xxx" default="orig_slot" %}
	{{ orig_slot }}
{% endfill %}

Above, I used the kwarg default="..." as a replacement for the as "..." syntax.

[EmilStenstrom]

Using default="orig_slot" looks much better! I think this is a good API.

[JuroOravec]

Wrote an MR for this at #504, @EmilStenstrom would you give it a look?