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.

Start by reproducing the ERC-7201-style example and tracing the existing handling of `.slot` for directly declared contract storage variables. The work is done when `$.fieldXXX.slot` is accepted for the shown storage reference and coverage verifies the generated storage-slot behavior.

Written by the indexing model from the issue text.