Apply suggestions from code review

Co-authored-by: Arman Bilge <armanbilge@gmail.com>

Author: FabioPinheiro

dom/src/main/scala/org/scalajs/dom/NDEFMessage.scala

@@ -15,5 +15,5 @@ import scala.scalajs.js.annotation.JSGlobal
 class NDEFMessage(recordsAgr: js.Array[NDEFRecordInit]) extends js.Object {
 
   /** Returns the list of NDEF records contained in the message. */
-  var records: FrozenArray[NDEFRecord] = js.native
+  def records: FrozenArray[NDEFRecord] = js.native
 }

dom/src/main/scala/org/scalajs/dom/NDEFReadingEvent.scala

@@ -12,11 +12,11 @@ import scala.scalajs.js
 trait NDEFReadingEvent extends Event {
 
   /** Returns an NDEFMessage object containing the received message. */
-  var message: NDEFMessage = js.native
+  def message: NDEFMessage = js.native
 
   /** Returns the serial number of the device, which is used for anti-collision and identification, or an empty string
     * if no serial number is available.
     */
-  var serialNumber: String = js.native
+  def serialNumber: String = js.native
 
 }

dom/src/main/scala/org/scalajs/dom/NDEFRecord.scala

@@ -14,25 +14,25 @@ class NDEFRecord(options: NDEFRecordInit) extends js.Object {
     * "empty", "text", "url", "smart-poster", "absolute-url", "mime", or "unknown" or else an external type name, which
     * consists of a domain name and custom type name separated by a colon (":").
     */
-  var recordType: String = js.native
+  def recordType: String = js.native
 
   /** Returns the MIME type of the record. This value will be null if recordType is not equal to "mime". */
-  var mediaType: js.UndefOr[String] = js.native
+  def mediaType: js.UndefOr[String] = js.native
 
   /** Returns the record identifier, which is an absolute or relative URL used to identify the record.
     *
     * Note: The uniqueness of the identifier is enforced only by the generator of the record.
     */
-  var id: js.UndefOr[String] = js.native
+  def id: js.UndefOr[String] = js.native
 
   /** Returns a DataView containing the raw bytes of the record's payload. */
-  var data: js.typedarray.DataView = js.native
+  def data: js.typedarray.DataView = js.native
 
   /** Returns the encoding of a textual payload, or null otherwise. */
-  var encoding: js.UndefOr[String] = js.native
+  def encoding: js.UndefOr[String] = js.native
 
   /** Returns the language of a textual payload, or null if one was not supplied. */
-  var lang: js.UndefOr[String] = js.native
+  def lang: js.UndefOr[String] = js.native
 
   /** Converts [[NDEFRecord.data]] to a sequence of records. This allows parsing the payloads of record types which may
     * contain nested records, such as smart poster and external type records.