Support `$.fieldXXX.slot` syntax for erc-7201 style storage

It'll be very helpful to support such syntax:

```solidity
Erc7201Storage storage $ = _getStorage();
assembly {
    sstore($.fieldXXX.slot, shr(192, calldataload(176)))
}
```

Currently, the `xxx.slot` syntax only works if `xxx` is a directly declared contract storage variable.