Check that we are in a list form when considering if we should drag pairs

[PEZ]

What has changed?

We were treating the binding box of a bound vector as if it was the binding form. Now we check that it is a list. I think this is actually just fixing the symptoms of something in the isPairs logic that is wrong, but the approach has worked for destructurings so far, so let's defer the real fix a while more...

Fixes #1914

My Calva PR Checklist

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Ping @PEZ, @bpringe, @corasaurus-hex, @Cyrik

[bpringe]

Looks good to me 🎉