Add pr_fixup action

[garritfra]

Adds an action that

a) sorts the entries alphabetically
b) adds back the newline delimiter in the array
c) formats the document
d) commits the changes if applicable

An example automated commit can be found here: 2a3d10d

This removes the need to check for accidentally committed whitespace, which is a bit hard to spot in the GitHub mobile app. Furthermore, we don't need the extra check for alphabetical order.