Add warning for overridden sidecar fields

[effigies]

trafficstars

https://github.com/bids-standard/bids-specification/pull/1834 proposes RECOMMENDING against overriding JSON values as a source of confusion ("I can see this value in my JSON, why is the tool using a different one?")

This should be relatively easy to implement by checking for overlap here: https://github.com/bids-standard/bids-validator/blob/9e8da1437b28bf9a69cce3a63727570ec719a754/src/schema/context.ts#L206-L207

Added a target: dev label to indicate PRs should target the dev branch, not main, as this would be a spec change.