Add the timestamp suffix to the format operator (#7292)

* Create timestamp.js

Taken verbatim from @ericshulman.

* Add description of the timestamp suffix

* Add an example for timestamp use

I also snuck it a couple of cosmetic corrections, because I was too lazy to open a new PR. 😢

* Correct example 4 to use the right date formats

* Correct description of the default date format

* Add a test for the timestamp suffix

* Add more format:timestamp tests

* Drop invalid input

* Update version tag in docs

---------

Co-authored-by: jeremy@jermolene.com <jeremy@jermolene.com>

core/modules/filters/format/timestamp.js

@@ -0,0 +1,25 @@
+/*\
+title: $:/core/modules/filters/format/timestamp.js
+type: application/javascript
+module-type: formatfilteroperator
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter function
+*/
+exports.timestamp = function(source,operand,options) {
+	var results = [];
+	source(function(tiddler,title) {
+		if (title.match(/^-?\d+$/)) {
+			var value = new Date(Number(title));
+			results.push($tw.utils.formatDateString(value,operand || "[UTC]YYYY0MM0DD0hh0mm0ss0XXX"));
+		}
+	});
+	return results;
+};
+})();