Add `is.formData()`

[Szymon Marczak]

From Got:

export default (body: unknown): body is FormData => is.nodeStream(body) && is.function_((body as FormData).getBoundary);

[Sindre Sorhus]

Hmm, not sure it's a good fit. Form data would be different between Node.js and the browser. That check is fine for Got, but this package works in both Node.js and the browser.