Add a JSON Validator for New Board PRs

[lorennorman]

We did this on the Components repo, here's the workflow file:
https://github.com/adafruit/Wippersnapper_Components/blob/main/.github/workflows/validate.yml

...and here's an example i2c-based schema:
https://github.com/adafruit/Wippersnapper_Components/blob/main/components/i2c/schema.json

...and pin-based schema:
https://github.com/adafruit/Wippersnapper_Components/blob/main/components/pin/schema.json

The goals is to reduce errors from manually writing JSON and help external users help themselves.

[lorennorman]

oops this is a dupe of #30, apologies