Add warning for overridden sidecar fields

[effigies]

bids-standard/bids-specification#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:

bids-validator/src/schema/context.ts

Lines 206 to 207 in 9e8da14

	this.sidecar = { ...json, ...this.sidecar }
	Object.keys(json).map((x) => this.sidecarKeyOrigin[x] ??= file.path)

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

[effigies]

This also relates to bids-standard/bids-2-devel#65, which would turn this from a warning to an error in BIDS 2.0.