add provers quorum to `easycrypt.project`

It would be nice to add the ability to set a quorum of provers in `easycrypt.project` files.

Of course, this can be done in individual theories, via
```
prover quorum=2 ["Alt-Ergo" "Z3" "CVC5"].
```

The rationale for quorums is building trust and proof stability.

The syntax could be
```
quorum = <n>
```
