fix:module field in pkg and update example

[bigbossx]

hello, @antfu @kazupon

I Struggling with the issue #14 #10 when i run example in this project.
finally,i think maybe the problem is the lastest version 0.2.2 use incorrect module field dist/index.mjs in package.json

webpack/webpack#7482

when we use dist/index.mjs .mjs tries to be as compatible as possible to node.js
so ⬇️

// dist/index.mjs
import VueCompositionApi from "@vue/composition-api" // this import incorrect  commonjs file

// dist/index.mjs
import { computed, getCurrentInstance } from "@vue/composition-api";
// node_modules/@vue/composition-api/index.js ❌

when we use dist/index.js

// dist/index.js
var _compositionapi = require('@vue/composition-api');
// node_modules/@vue/composition-api/dist/vue-composition-api.mjs 🌈

finally

so i modify something in package.json follow this link https://tsup.egoist.sh/#bundle-formats

is my understanding correct, maybe I'm missing something?

[bigbossx]

This only seems to happen with webpack4（use by vue-cli）, I tested webpack2 with webpack5 and there is no problem with confusing module references. Also .mjs is probably an important way to distinguish module types in the future, so maybe my pr is a pointless fix, so close

for webpack4 we can fix it by
vuejs/composition-api#837

[kingyue737]

Not a pointless fix! This package does not work in vitest as it doesn't support dual-packaging correctly, ref https://github.com/sheremet-va/dual-packaging.

I think you should reopen this PR