indenters: Add vala indenter

This adds these things:

• Buildsystem-Plumbing for allowing more indenters
• An option for using it in test-widget (Albeit I can reverse this one easily)
• Indenter for Vala
• A few unit tests for the indenter (Implemented a bit hacky, as I didn't find a way that does not involve dozens of files)

This MR is a draft because of these issues:

• A lot of unnecessary allocations+a bit of codestyle probably
• Maybe a few missing unit tests

Would resolve gnome-builder#1662 (closed) as the pipeline-addin is already ported.

(I will remove the changes to the test-widget as soon as it is ready to land)