Parse ``immutable`` as data location.

First step in https://github.com/ethereum/solidity/issues/13323.

Previously, we considered adding a ``code`` data location - however, since EOF will have an actual data section, ``code`` as keyword may actually be misleading. So we intend to stick with more semantic term ``immutable``, but change it from only a qualifier for state variables to a full data location as well.

Start by reading the linked issue #13323 and the discussion of EOF's data section, the proposed `immutable` data location, and why `code` is misleading. Trace how data locations and the `immutable` state-variable qualifier are parsed in the Solidity compiler. Done means `immutable` works as the intended full data location without breaking its existing qualifier use, with the relevant parser and semantic behavior covered.

Written by the indexing model from the issue text.