Fix exports: ESM & CJS multi-module

[pubkey]

This PR changes the build scripts so that this package can be used either as esm module or as cjs package at the same time.

This fixes #21 while making it still usable for everyone who is still on cjs modules.

Questions:

• Would you accept a PR to add a CI via github actions with some tests?
• Would you accept a PR to add typescript typings (or even rewrite the code to typescript)?
• Would you accept a PR to have a node.js fallback where the global window variable is not defined?

[zalo]

@gfodor What do you think of this one?