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>;
}