feat: send media as stickers (#479)

Adds the option `sendMediaAsSticker` that will take care of converting media to appropriate formats and send it as a sticker. Note that ffmpeg is required to properly convert animated stickers that are not in webp format.

Co-authored-by: Pedro S. Lopez <pedroslopez@me.com>

index.d.ts

@@ -277,6 +277,9 @@ declare namespace WAWebJS {
         /** User agent to use in puppeteer.
          * @default 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/72.0.3626.109 Safari/537.36' */
         userAgent?: string
+        /** Ffmpeg path to use when formating videos to webp while sending stickers 
+         * @default 'ffmpeg' */
+        ffmpegPath?: string
     }
 
     /** Represents a Whatsapp client session */
@@ -553,6 +556,8 @@ declare namespace WAWebJS {
         linkPreview?: boolean
         /** Send audio as voice message */
         sendAudioAsVoice?: boolean
+        /** Send media as sticker */
+        sendMediaAsSticker?: boolean
         /** Send media as document */
         sendMediaAsDocument?: boolean
         /** Automatically parse vCards and send them as contacts */