Update to ES modules

[robatwilliams]

Update the code of the plugin itself to use import and export rather than require and module.exports.

Update the lint rules in eslintrc to require this style, and prohibit CommonJS style.

[robatwilliams]

Runs into problem https://github.com/mysticatea/eslint-plugin-node/issues/349

Could disable that rule, and rely on CI unit tests to catch e.g. Node 14 issues. But currently that doesn't cover the check-es-compat utility.

[robatwilliams]

Disable the rule, and use es-compat itself to check compatibility of its own source code? (!) But that would be disconnected from package.json:engines

[robatwilliams]

Need to switch to https://github.com/eslint-community/eslint-plugin-n because the author (same one as eslint-plugin-es) has also abandoned his node plugin

[robatwilliams]

Blocked on Jest ESM support - https://jestjs.io/docs/ecmascript-modules

[robatwilliams]

to do as part of this eventually:

• remove mjs extension and references to it
• type:module in package.jsons
• import/newline-after-import instead of custom spacing lint rule
• file extensions are required with ES modules
• enable specific recommended config plugin:n/recommended-module
• sourceType=module in parser options
• enable eslint import plugin /recommended

[robatwilliams]

Also see this about importing browser compat data https://github.com/mdn/browser-compat-data/releases/tag/v5.0.0