Add missing API for underscore

Add utility function unescape()

underscore/underscore.d.ts

@@ -1442,6 +1442,13 @@ interface UnderscoreStatic {
 	**/
 	escape(str: string): string;
 
+	/**
+	* The opposite of escape, replaces &, <, >, ", and ' with their unescaped counterparts.
+	* @param str HTML escaped string.
+	* @return `str` Raw string.
+	**/
+	unescape(str: string): string;
+
 	/**
 	* If the value of the named property is a function then invoke it; otherwise, return it.
 	* @param object Object to maybe invoke function `property` on.
@@ -2283,6 +2290,12 @@ interface Underscore<T> {
 	**/
 	escape(): string;
 
+	/**
+	* Wrapped type `string`.
+	* @see _.unescape
+	**/
+	unescape(): string;
+
 	/**
 	* Wrapped type `object`.
 	* @see _.result
@@ -3106,6 +3119,12 @@ interface _Chain<T> {
 	**/
 	escape(): _Chain<T>;
 
+	/**
+	* Wrapped type `string`.
+	* @see _.unescape
+	**/
+	unescape(): _Chain<T>;
+
 	/**
 	* Wrapped type `object`.
 	* @see _.result