Add safe schema change for descriptions changing

[JoviDeCroock]

This adds a new safe schema-change when a description changes of any schema-member. Do note that there are some drive-by fixes in here as I noticed a bug in safe-argument changes where we'd just always report the changes rather than actually see whether something changed 😅

Resolves #4245

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[yaacovCR]

as I noticed a bug in safe-argument changes where we'd just always report the changes rather than actually see whether something changed

I broke this out into a separate commit on this fix out into a separate commit on this new branch: https://github.com/graphql/graphql-js/tree/break-out-fix

Instead of doing stacked PRs, we can just have multiple commits within a PR and use the "rebase" merge strategy instead of the "squash" strategy. This makes the git history easier to follow in the long run but doesn't lead to (as much) rebasing madness.

[yaacovCR]

Left a comment with something to consider, and addressed separately the drive-by changes. I guess we should try our best to minimize those, I know I've been tempted to sneak them in when possible!