Type-Checking for ``immutable`` as data location.

Depends on https://github.com/ethereum/solidity/issues/13368.
As a second step of https://github.com/ethereum/solidity/issues/13323, we properly type check the new ``code`` data location.

This entails removing the syntax error introduced in https://github.com/ethereum/solidity/issues/13368 and instead implementing the type checker logic for the ``code`` data location.

This will mostly involve conscious copying of the behaviour of the ``calldata`` location in runtime context and of the ``memory`` location in creation context. Since ``code`` variables behave differently depending on creation vs runtime context, this will require some refactoring of the type checking logic (may overlap with https://github.com/ethereum/solidity/issues/13365).

This should be accompanied by extensive syntax tests that demonstrate the type checking behaviour.

Finally, in code generation, any attempt to generate code for a ``code`` variable should result in an ``solUnimplementedAssert`` to be eliminated in subsequent steps.

Start by reading issues 13368, 13323, and 13365, then compare the existing type-checking behavior for calldata and memory in runtime and creation contexts. Review the syntax-test coverage and code-generation path described in the issue; done means code-location type checking is implemented, syntax tests cover it, and code generation emits solUnimplementedAssert for code variables.

Written by the indexing model from the issue text.