Improve portability of the UMD build

[chocolateboy]

This PR:

• replaces self with globalThis
• makes detection of CommonJS environments more robust

globalThis is the official way to refer to the global scope and is supported by almost all environments:

engine	self	globalThis
Node.js	✖	✔
QuickJS	✖	✔
SpiderMonkey	✖	✔
Modern browsers	✔	✔
Deno	✔	✔
IE11	✔	✖

The notable exception is IE11, which will need to use a polyfill. If IE11 support was previously promised or assumed, then this would be a breaking change.

[davidchambers]

Thanks for the pull request, @chocolateboy! I pushed a few tweaks to your branch. Are you happy with them?

[chocolateboy]

Yes, LGTM, thanks!