Adjust Fetch Polyfill

[sxzz]

Fetch API now is already supported by modern browsers and Node.js >= 18.

For browsers, we can completely remove the polyfills.

For the node version (cjs and esm), we can replace fetch-undici (which is deprecated) with undici. They're the same but undici is the latest one. Also, we can also completely remove polyfills after 2023-09-11 (Which is the EOL of Node 16).

/cc @Songkeys

[Songkeys]

fetch-undici is not deprecated yet. It's the isomorphic exports of fetch. We can directly use undici but have to bundle it correctly for browser.

I agree we can remove it, actually even before v16 EOL. Let's do it in the next version (v1).

[sxzz]

Oh, there are actually two similar libraries undici-fetch and fetch-undici. I was wrong about these two.

[sxzz]

Is there a reason to do it in the next major version? Since for 0.x.x, we can release breaking changes in a minor version.

[Songkeys]

We can but we don't have to since it doesn't improve what matters a lot. I'm thinking about starting v1 very soon. 0.19.x might be the last major version. But yes, we can.