Consider removing UMD bundle

[ptomato]

Suggestion from a conversation with @43081j, who said that most users will fall into one of these 3 groups:

• Browser
• node <=18 and/or no type: "module" in their package.json
• node >18 and/or type: "module" in their package.json

The first and third of these groups can use the ES module bundle, while the second group requires a CommonJS bundle. We may only need the UMD bundle if we really need to support ES5. That was certainly a requirement when we started this polyfill, but might not be anymore.

[43081j]

i should reword this a little. ESM can be loaded by node <=18, but it cannot be required (require(esm))

in node 20 and above, you can require an ES module. you can't do this in older versions.

so basically the CJS bundle is only useful for people who are in a CJS project (no type: "module") on a node version before 20.