[5.x] Provide Site attribute helper

[ajnsn]

When using Antlers, accessing Site attributes is straightforward. For example, you can use:
{{ site:attributes:company_name }}

In Blade, the equivalent is often done like this?
{{ $site->attributes()['company_name'] }}

However, if the attribute isn't set, this will throw an "Undefined array key" exception. Using Arr::get to handle this is an option, but it adds a lot of overhead.

This pull request introduces a simple attribute helper:
{{ $site->attribute('company_name') }}

As a bonus, this helper "supports" dot notation and allows you to provide a fallback value.