fix: importing from esm in node

[chearon]

Right now you can't import {watch} from "@vue/composition-api" in nodejs from a .mjs file. If you try that, you'll get:

file:///Users/caleb/Code/temp/x.mjs:1
import {ref} from "@vue/composition-api";
        ^^^
SyntaxError: Named export 'ref' not found. The requested module '@vue/composition-api' is a CommonJS module, which may not support all module.exports as named exports.

I think since there's no "type" in package.json, the default module type for any .js is CJS. The files in the red and the green appear to be identical so I think this is low-risk and the right way to do it. As I understand it Webpack will parse anything, it doesn't really have a concept of module type, and not sure about other bundlers.