fixes#22791
This pull request introduces a minor improvement to the handling of
immutable variables in the compiler and adds a new test case for nested
case objects. The most important changes are:
### Compiler improvements
* Updated the `isLet` guard in `compiler/guards.nim` to recognize
`skConst` symbols as immutable variables, ensuring that constants are
correctly identified alongside lets and other immutable types.
### Test coverage
* Added a new test in `tests/objvariant/tcorrectcheckedfield.nim` for
bug #22791, verifying correct pattern matching and field access in
nested `case` objects with constants.
fixes#24021
The field checking for case object branches at some point generates a
negated set `contains` check for the object discriminator. For enum
types, this tries to generate a complement set and convert to a
`contains` check in that instead. It obtains this type from the type of
the element node in the `contains` check.
`buildProperFieldCheck` creates the element node by changing a field
access expression like `foo.z` into `foo.kind`. In order to do this, it
copies the node `foo.z` and sets the field name in the node to the
symbol `kind`. But when copying the node, the type of the original
`foo.z` is retained. This means that the complement is performed on the
type of the accessed field rather than the type of the discriminator,
which causes problems when the accessed field is also an enum.
To fix this, we properly set the type of the copied node to the type of
the kind field. An alternative is just to make a new node instead.
A lot of text for a single line change, I know, but this part of the
codebase could use more explanation.
