Add method BlobStream.seek

[asfernandes]

export const enum BlobSeekWhence {
    START = 0,
    CURRENT = 1,
    END = 2
}

/** BlobStream class. */
export abstract class BlobStream {
    /** Seeks into the blob stream and return the new position. */
    abstract seek(offset: number, whence?: BlobSeekWhence): Promise<number>;
}