chore: enable importing module in node

[danielroe]

This line was originally added by #70 (at which point there were no mjs/cjs files).

Now that there are, this line is now preventing importing the .mjs file from within a module context in node.

The error generated is:

import { isRef, ref, watchEffect, computed, customRef, unref, inject, watch, getCurrentInstance, onMounted, onUpdated, shallowRef, getCurrentScope, reactive, markRaw, readonly, onBeforeUpdate, isVue2 } from 'vue-demi';
                ^^^
SyntaxError: Named export 'ref' not found. The requested module 'vue-demi' is a CommonJS module, which may not support all module.exports as named exports.
CommonJS modules can always be imported via the default export, for example using:

import pkg from 'vue-demi';
const { isRef, ref, watchEffect, computed, customRef, unref, inject, watch, getCurrentInstance, onMounted, onUpdated, shallowRef, getCurrentScope, reactive, markRaw, readonly, onBeforeUpdate, isVue2 } = pkg;