Target older ES version

[CodingFabian]

I am trying to get a next + sanity working on Windows 7 Firefox ESR 78. Yes that setup is old, and maybe you don't want to do anything about it.
It currently fails with

private fields are currently not supported

which is as far as I can see only used in fetchXhr.ts - Would you be willing to make this ES5 compatible?

[CodingFabian]

Alternatively users can transpile with babel:

					['@babel/plugin-transform-private-property-in-object', { loose: true }],

Which is what I am doing right now.

[rexxars]

If you're using this with the Sanity client, my assumption is you would run into similar issues with that as well, once the changes in this repo has been addressed.