fix: mark @vue/compiler-sfc as an optional peer dependency

Fixes vuejs/vue-cli#6595
Fixes vuejs/vue-cli#6596

It's not optional actually.

However, in Vue CLI projects for Vue 2, vue-loader v16 is installed but
not used. In that case, the missing peer dependency error should not be
thrown.

There's a dependency check in the entry file anyway.

package.json

@@ -40,6 +40,11 @@
     "@vue/compiler-sfc": "^3.0.8",
     "webpack": "^4.1.0 || ^5.0.0-0"
   },
+  "peerDependenciesMeta": {
+    "@vue/compiler-sfc": {
+      "optional": true
+    }
+  },
   "devDependencies": {
     "@babel/core": "^7.7.7",
     "@babel/preset-env": "^7.11.5",