Add APKBUILD to be highlighted as a Bash file

Summary: APKBUILDs are build scripts used by Alpine Linux and it's derivatives, much like PKGBUILDs from Arch Linux. They use the Bash syntax so it only makes sense to highlight them by default.

Test Plan: Just open any APKBUILD (for example this one) and check if the highlighting is done properly.

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: dhaumann, bshah, vkrause, bcooksley, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10072