Refactor to update internals for TS7

Closes GH-21.

Reviewed-by: Remco Haszing <remcohaszing@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Author: sandersn

lib/index.js

@@ -158,7 +158,6 @@ export class VFileMessage extends Error {
         ? options.place.start
         : options.place
 
-    /* eslint-disable no-unused-expressions */
     /**
      * Stack of ancestor nodes surrounding the message.
      *
@@ -196,7 +195,7 @@ export class VFileMessage extends Error {
      *
      * @type {string | undefined}
      */
-    this.file
+    this.file = ''
 
     // Field from `Error`.
     /**
@@ -274,21 +273,21 @@ export class VFileMessage extends Error {
      *
      * @type {string | undefined}
      */
-    this.actual
+    this.actual = undefined
 
     /**
      * Suggest acceptable values that can be used instead of `actual`.
      *
      * @type {Array<string> | undefined}
      */
-    this.expected
+    this.expected = undefined
 
     /**
      * Long form description of the message (you should use markdown).
      *
      * @type {string | undefined}
      */
-    this.note
+    this.note = undefined
 
     /**
      * Link to docs for the message.
@@ -298,8 +297,7 @@ export class VFileMessage extends Error {
      *
      * @type {string | undefined}
      */
-    this.url
-    /* eslint-enable no-unused-expressions */
+    this.url = undefined
   }
 }