Drop CommonJS support in Node packages, use `"type": "module"`

[nolanlawson]

There is at least one dependency in our Node packages (e.g. estree-walker, which is a dependency of @lwc/template-compiler), which we cannot currently update because it dropped support for CommonJS.

Unfortunately ESM is kind of viral – if you want to import ESM synchronously, you basically need to use "type": "module" yourself. So updating our packages to "type": "module" would be a breaking change for our consumers.