Added Bulgarian Eloquent JavaScript (#4999)

* Added Bulgarian Eloquent JavaScript

Added the Bulgarian translation of Eloquent JavaScript (title is untranslated in the Bulgarian book)

* Removed trailing slash

free-programming-books-bg.md

@@ -38,6 +38,7 @@
 
 ### JavaScript
 
+* [Eloquent JavaScript](https://to6esko.github.io) - Marijn Haverbeke (HTML)
 * [Основи на програмирането с JavaScript](https://js-book.softuni.bg) - Светлин Наков и колектив