Parse ``immutable`` as data location.

[ekpyron]

First step in #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.

[ekpyron]

When closing this issue, #13369 should be moved to the Design/Decide column of the Focus Board.