unbound named parameters in mapping types

[OmarTawfik]

For a mapping type like this:

    mapping(uint256 proposalId => ProposalCore) private _proposals;

The identifier proposalId is not bound to anything, while it should be bound to a new definition, similar to parameter names.

Just skimming through the relevant links below, it seems like its scope is restricted to only the mapping(...), this it cannot be referenced anywhere else for now, but we should double check this:

• https://soliditylang.org/blog/2023/02/01/solidity-0.8.18-release-announcement/
• Allow specifying parameter name in public mappings ethereum/solidity#11407
• Allow named parameters in mapping types ethereum/solidity#13384