chore: mark @vue/compiler-dom as optional (#1958)

This should solve the pnpm error:

```
├─┬ @vue/test-utils 2.2.8
│ └── ✕ missing peer @vue/compiler-dom@^3.0.1
```

Author: cexbrayat

package.json

@@ -67,6 +67,9 @@
     "@vue/compiler-dom": "^3.0.1",
     "vue": "^3.0.1"
   },
+  "optionalDependencies": {
+    "@vue/compiler-dom": "^3.0.1"
+  },
   "author": {
     "name": "Lachlan Miller",
     "email": "[email protected]"