refactor: drop vue-demi dependency (for now)

using vue-demi requires that vue2.6 projects install "@vue/composition-api", always! Even though I believe is the norm in 2.6.x projects but still kind of bummer to get an install error. vue-demi lists @vue/composition-api as optional peer dependency because it would otherwise install it for vue2.7 and vue3.x projects. We could list @vue/composition-api as dependency as well to overcome the install issue, but we're not using anything on this package so why complicate things?

1. I've removed vue-demi altogether, replaced the only thing we've use isVue3 with simply version regexp
1. the swap-vue.js from vuelidate allow us to keep the isomorphic testing to ensure it works everything

Related issues:
vueuse/vue-demi#67
vueuse/vue-demi#7

package.json

@@ -18,8 +18,8 @@
     "format:staged": "pretty-quick --staged",
     "release": "np",
     "version": "npm run build",
-    "use-vue:2": "node scripts/swap-vue.js 2 && vue-demi-switch 2",
-    "use-vue:3": "node scripts/swap-vue.js 3 && vue-demi-switch 3"
+    "use-vue:2": "node scripts/swap-vue.js 2",
+    "use-vue:3": "node scripts/swap-vue.js 3"
   },
   "repository": {
     "type": "git",
@@ -68,16 +68,7 @@
   "engines": {
     "node": ">=6"
   },
-  "dependencies": {
-    "vue-demi": "^0.13.2"
-  },
   "peerDependencies": {
-    "@vue/composition-api": "^1.0.0",
     "vue": "^2.6.0 || >=3.2.0"
-  },
-  "peerDependenciesMeta": {
-    "@vue/composition-api": {
-      "optional": true
-    }
   }
 }

src/utils.js

@@ -1,4 +1,6 @@
-import { isVue3 } from 'vue-demi'
+const version = require('vue').version || require('vue').default.version
+
+export const isVue3 = /^3\./.test(version)
 
 // eslint-disable-next-line
 export const directiveHooks = {

test/test-utils.js

@@ -1,4 +1,4 @@
-const { isVue3 } = require('vue-demi')
+const { isVue3 } = require('../src/utils')
 
 const lifecycleInstanceMethods = {
   unmount: isVue3 ? 'unmount' : 'destroy',