FR: add a way to suppress warning on migration dropping field

[crusso]

WARN: /home/runner/work/motoko-benchmark/motoko-benchmark/benchmarks/application/tmp/car-sharing-upgrade/main.mo:11.7-11.32: warning [M0207], migration expression consumes field `minCarCapacity` of type
  Nat
but does not produce it. The field is not declared in the actor.
This field will be removed from the actor, discarding its consumed value.
If this removal is unintended, declare the field in the actor and either remove the field from the parameter of the migration function or add it to the result of the migration function.

There is no way to suppress this warning, even if discard is intentional.

We could maybe look for an explicit comment on the field to suppress the warning.

/// @deleted
var x : Nat (edited)

or use a specially named type

var x : (deleted: Nat)

Suppressing all these warnings is probably a bad idea, since some data may be unintentionally lost.