Changes to Jessie await lint rules

[michaelfig]

@erights writes in Agoric/agoric-sdk#7870 (comment):

I do think it is worth making both of these changes to Jessie.

IMO the @jessie.js/no-nested-await should default to 'off', but would like your thoughts.
IF @jessie.js/no-nested-await is downgraded from error, then @jessie.js/safe-await-separator should be upgraded to error. ELSE no change suggested.
But since Jessie is a separate repo, this PR does not need to wait on that Jessie change.