build: use cjs/mjs extensions for cjs/esm builds

[danielroe]

Summary

This PR proposes using .cjs and .mjs instead of the current .js extensions. That will allow usage of this library in a native Node module context.

old	new
vue-router.cjs.js	vue-router.cjs
vue-router.cjs.prod.js	vue-router.prod.cjs
vue-router.esm-bundler.js	vue-router.mjs

Notes

• As is, this is a breaking change as it changes the packaging expectations. I've added stubs that re-export these .mjs/.cjs files for backwards compatibility.
• Because this package imports vue, vue will also need to provide appropriately named native Node module filenames. (This should not negatively affect existing users who will not be using this module natively anyway.)

Background issue

At the moment, importing vue-router within an ESM context produces the following error:

(node:73025) Warning: To load an ES module, set "type": "module" in the package.json or use the .mjs extension.
(Use `node --trace-warnings ...` to show where the warning was created)
Server Side Rendering Error: /project/node_modules/vue-router/dist/vue-router.esm-bundler.js:6
import { getCurrentInstance, inject, onUnmounted, onDeactivated, onActivated, computed, unref, watchEffect, defineComponent, reactive, h, provide, ref, watch, shallowRef, nextTick } from 'vue';
^^^^^^

SyntaxError: Cannot use import statement outside a module
    at wrapSafe (internal/modules/cjs/loader.js:1001:16)
    at Module._compile (internal/modules/cjs/loader.js:1049:27)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1114:10)
    at Module.load (internal/modules/cjs/loader.js:950:32)
    at Function.Module._load (internal/modules/cjs/loader.js:790:14)
    at ModuleWrap.<anonymous> (internal/modules/esm/translators.js:199:29)
    at ModuleJob.run (internal/modules/esm/module_job.js:169:25)
    at async Loader.import (internal/modules/esm/loader.js:177:24)
    at async __instantiateModule__ (file:///project/playground/.nuxt/dist/server/server.mjs:1844:3)

[posva]

Thanks a lot for this!

I think we should align with Vue. Regarding the changes, it's IMO okay to release this in 4.1.0 with information about the changes necessary for the user.

[posva]

Published and testable under the egde tag, vue-router@edge

[danielroe]

A repo with vite & vue cli examples using the published version: https://github.com/danielroe/vue-esm-test and https://github.com/danielroe/vue-esm-test/tree/vue-cli.