Refactoring the global object initialization checker (#17996)

This PR refactors the global object initialization checker as
implemented in
`compiler/src/dotty/tools/dotc/transform/init/Objects.scala` with the
following:

1. Refactoring the type hierarchy for abstract values by adding a new
base class `ValueElement` which has `Cold` and `Ref` as child classes.
This separates the first-order abstract values (`Cold` and `Ref`) from
second-order abstract values (`Refset`)
2. Adding documentation for the data structures and abstract syntax used
in the initialization checker
3. Adding return handlers which removes the warning for return
statements in secondary constructors, which is shown in
`tests/init-global/pos/secondary-constructor-return.scala`