Convert package to ESM

[nzakas]

To convert this package from CommonJS to ECMAScript modules (ESM):

• Add type: module to package.json
• Convert files in lib to ESM
• Convert files in tests/lib to ESM
• Upgrade Mocha
• Create build script in package.json to generate CommonJS file (.cjs) in dist directory (use Rollup)
• Add dist to .gitignore
• Run build in prepare script in package.json to ensure build is run at key moments
• Update package.json so that main points in the CJS file in dist and exports provides both require and import options
• Update README instructions

[brettz9]

Would you like to change to named exports here too?

[nzakas]

Sure, I'm open to that.

[brettz9]

FWIW, in my PR I've kept some single item files as default exports, but for the main one (the one being exported) and others, they are named.

[aladdin-add]

as I mentioned here, eslint-scope was used by webpack, too. the change would stop webpack upgrading to the latest eslint-scope. // @sokra

[nzakas]

@aladdin-add would it stop them from upgrading, or would they just wait a bit longer? I assume with a breaking change that it would probably go through a more thorough evaluation before upgrading regardless.

[aladdin-add]

yes, they will drop Node.js < 12 at some point, it's just a bit longer.