push rule: check that .sh files have the 'x' flag set correctly

Description

(Include problem, use cases, benefits, and/or goals)

Proposal

Add a push rule to check that .sh (or other executables) have the 'x' flag set

Links / references

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem. Developers can forget doing a chmod +x
2. What is the solution. do a check for the 'x' flag on executable files (or alternatively set them automatically?)
3. How does someone use this push rules

During implementation, this can then be copied and used as a starter for the documentation.)