Add JSON validation to GitHub Action

[DHardy-WMF]

Description

This is a follow-up patch for T404882. A new step was added to prevent invalid JSON syntax in pull requests, which is determined by a non-zero exit code returned from the jq command.

Rationale

Prevents junk syntax from being added to automated pull requests.

Phabricator Ticket

https://phabricator.wikimedia.org/T404882

How Has This Been Tested?

Forked branch

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Types of changes

What types of changes does your code introduce? Add an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Minor change (fix a typo, add a translation tag, add section to README, etc.)