Generate `toBuilder` overrides with covariate return type

At the moment `toBuilder` methods are only defined on the `GeneratedMessage`, which means they are always of type `GeneratedMessage`. This works poorly when `no-implicit-casts` is used as it requires explicit casting to the actual type.

No files or tests are named. Start by locating the Dart generator and the GeneratedMessage definition, then trace how toBuilder is currently declared and emitted. Done means generated message classes provide toBuilder overrides with covariant concrete return types, avoiding explicit casts when no-implicit-casts is enabled.

Written by the indexing model from the issue text.