chore: Convert project to us ECMAScript modules

[jdufresne]

ECMAScript modules are the official standard format to package JavaScript code for reuse. Modules are defined using a variety of import and export statements. Node fully supports ECMAScript modules as they are currently specified and provides interoperability between them and its original module format, CommonJS.

More and more Node packages are switching from CommonJS to ESM, including SVGO dependencies such as csso.

Node documentation on ESM is available at:
https://nodejs.org/api/esm.html

[SethFalco]

Related: #1429

Thanks for dedicating the time to perform the migration. This is something we knew had to be done eventually and was even one of the goals for SVGO v3, but was never prioritized. I'd also been putting it off.

I'll actually put the performance/test work on-hold to review these incoming pull requests (I can still execute them locally anyway). We're starting to get a lot of pull requests piling up now, plus it'll be great to work with modules instead.

Reference: Progress on performance and expanded regression testing.

I would like to do one more release with CommonJS with the improvements we've been making recently, and some optimizations for existing plugins Kendell R has been working on.

We'll get this reviewed and merged after that, and prep for SVGO v4. 🎉

Meanwhile, I'd be happy to assist you with rebasing as we merge other pull requests. 👍🏽

[jdufresne]

Sounds great. Thanks!

[SethFalco]

I've made the following changes:

• Rebased with main.
• Removed use strict from all files, as that is implicit in modules.
• Updated our Rollup config to also package a CJS bundle to preserve CommonJS compatibility.

The entire contents of JavaScript modules are automatically in strict mode, with no statement needed to initiate it.

— https://developer.mozilla.org/docs/Web/JavaScript/Reference/Strict_mode#strict_mode_for_modules

I've seen many package maintainers drop CJS support when they move to ESM, so I assumed that maintaining both would be a challenge. After looking into it, it's actually not a problem at all as we're a stateless package. It does come with the downside of increasing the package size, but that's worth it to support a still substantial portion of the Node.js ecosystem. 🤔

We'll get this reviewed and merged after that, and prep for SVGO v4. 🎉

For now, I'd like to support both CJS and ESM. With this in mind, it's probably better to save SVGO v4 for another time, as this isn't the breaking change I was expecting it to be. I'd like to continue resolving bugs in the current version.

Later this week I'll skim over the changes again, verify that it's working properly in both ESM, CJS, CLI, and browser, and potential add tests specific to the CJS module similar to what we have with the browser bundle. Then I'll be glad to merge this!

Please let me know if you think there are any issues with my changes.