Add configuration validation for renovate.json

[emmileaf]

Look into a way to add check to verify renovate.json in presubmit, so things like #1349's missed comma can be caught ahead of renovate bot issue.

• I had missed verifying with these steps when making the PR changes: https://github.com/renovatebot/renovate/blob/main/docs/usage/getting-started/installing-onboarding.md#reconfigure-via-pr

• The renovate-config-validator can probably be added as a step in an existing presubmit action, or as a pre-commit hook: https://github.com/renovatebot/pre-commit-hooks

[blakeli0]

It happened again, we didn't notice the issue until a few months later, #2374

[emmileaf]

Bleh this totally fell off my radar last year, sorry 🥲 Thank you @ddixit14 for figuring it out! 🙏

[diegomarquezp]

cc: @burkedavison @lqiu96
I'm planning to do a basic parsable JSON test and the use https://docs.renovatebot.com/config-validation/#config-validation

edit: maybe the parsable JSON check is included in this tool

[diegomarquezp]

This was already fixed by #2404