Use single file for extends

[jgthms]

This is a bugfix.

Proposed solution

Instead of importing the mixins file which also includes all Sass placeholders, move all placeholders to its own extends file, and import that file only once.

Also, all references to the .sass file extension have been removed, since they're unnecessary when there's no ambiguity between a .sass file or a .scss file.

Tradeoffs

If you imported mixins to get the placeholders, you need to include extends instead.
If you imported using .sass, you'll need to remove it now.

Testing Done

Imported Bulma in custom setups. Still works.

Changelog updated?

No.