fix(storage): use backward compatible return type in download function

[itslenny]

🔍 Description

Update BlobDownloadBuilder to be fully compatible with the Promise interface to maintain backward compatibility.

What changed?

BlobDownloadBuilder now implements the entire Promise interface so it can be assigned to a Promise variable without TypeScript complaining.

Why was this change needed?

This resolves a breaking type change introduced in storage-js 2.12.2 by this PR: supabase/storage-js#137

Currently, assigning the return of the download() function to a strongly typed variable results in a type error:

Argument of type 'BlobDownloadBuilder' is not assignable to parameter of
type 'Promise<StorageApiResponse<unknown>>'.

Type 'BlobDownloadBuilder' is missing the following properties from
type 'Promise<StorageApiResponse<unknown>>': catch, finally, [Symbol.toStringTag]

Additionally, attempting to use .catch() or .finally() would result in an error as those functions were not defined in BlobDownloadBuilder so anyone depending on them would hit a runtime error.

📸 Screenshots/Examples

[coveralls]

coverage: 95.455% (+13.0%) from 82.5%
when pulling 4f80484 on fix/make-download-return-type-backward-compatible
into 00c4a11 on master.