fix: Add stub ReadableStream declaration

[LinusU]

The patch here will make this library usable without including the dom typings, which usually is not desirable in a pure Node.js project.

This is the same approach used by the AWS SDK, ref: aws/aws-sdk-js-v3#6554

Without this, there is an error when trying to use this package (or in my case, the got package) in Node.js:

node_modules/form-data-encoder/lib/index.d.ts:30:15 - error TS2304: Cannot find name 'ReadableStream'.

30     stream(): ReadableStream<Uint8Array> | AsyncIterable<Uint8Array>;
                 ~~~~~~~~~~~~~~


Found 1 error in node_modules/form-data-encoder/lib/index.d.ts:30

[octet-stream]

Hello, thanks for your contribution.
The issue was already discussed here and here. As I said, my library is not the right place for the fix. Please open a PR to https://github.com/DefinitelyTyped/DefinitelyTyped instead and add proper typings for @types/node package.

[octet-stream]

There's also discussion in DefinitelyTyped repo about this problem, still w/o any response: DefinitelyTyped/DefinitelyTyped#69452

[LinusU]

Turns out that this was fixed by https://github.com/DefinitelyTyped/DefinitelyTyped/pull/70269/files

Sorry for the noise...