### What
Continuation of #7355 by extending the functionality to named tabs.
Updates `flattenFields` to hoist nested fields inside named tabs to the
top-level field array when `moveSubFieldsToTop` is enabled.
Also fixes an issue where group fields with custom cells were being
flattened out.
Now, group fields with a custom cell components remain available as
top-level columns.
Fixes#12563
20f7017758