Fix to revert to ducktyping

* Before this major, files were ducktyped.
* With this major, VFiles across realms (e.g., from multiple install locations)
  were wrapped, causing state to be lost
* This fixes that

lib/index.js

@@ -40,7 +40,7 @@ export function toVFile(options) {
     options = {path: fileURLToPath(options)}
   }
 
-  return options instanceof VFile ? options : new VFile(options)
+  return looksLikeAVFile(options) ? options : new VFile(options)
 }
 
 toVFile.readSync = readSync
@@ -208,3 +208,16 @@ toVFile.write =
       }
     }
   )
+
+/**
+ * @param {Compatible} value
+ * @returns {value is VFile}
+ */
+function looksLikeAVFile(value) {
+  return (
+    value &&
+    typeof value === 'object' &&
+    'message' in value &&
+    'messages' in value
+  )
+}

package.json

@@ -56,7 +56,7 @@
     "format": "remark . -qfo && prettier . -w --loglevel warn && xo --fix",
     "test-api": "node test.js",
     "test-coverage": "c8 --check-coverage --branches 100 --functions 100 --lines 100 --statements 100 --reporter lcov node test.js",
-    "test": "npm run format && npm run test-coverage"
+    "test": "npm run build && npm run format && npm run test-coverage"
   },
   "prettier": {
     "tabWidth": 2,