Add icon-address function

Adding this missing icon function that can help make building icon fonts more easy.

For example this will be used to build icon components by looping through an $icon-list.

Status: **Opened for visibility**
Reviewers: @kpeatt @ry5n

## Changes
- Add `_icon-address.scss` file
- Import `icon-address` into `_spline.scss`

Hmm, I dunno, isn't the `@if (map-has-key())` part already kind of doing that?

Or more specifically, it errors out if `$icon-list` isn't a map or if it doesn't exist so would a `variable-exists()` do anything?

I suppose it could be used to avoid the error itself and not block the CSS from rendering?