Function newLegacyFieldReader

newLegacyFieldReader(): {
    dropped: StoredSnapshotFieldDropped[];
    read: <A>(
        field: string,
        schema: Decoder<A>,
        value: unknown,
    ) => A | undefined;
}
The reading of one stored document's legacy fields: each is read with its release's own schema, and one that does not decode is listed in dropped and read as absent.

Returns {
    dropped: StoredSnapshotFieldDropped[];
    read: <A>(
        field: string,
        schema: Decoder<A>,
        value: unknown,
    ) => A | undefined;
}
- dropped: StoredSnapshotFieldDropped[]
- read: <A>(field: string, schema: Decoder<A>, value: unknown) => A | undefined
  A legacy field read with its own historical schema; undefined when absent or not understood.