feat(adding extension example for merging explore queries by url)

[LukaFontanilla]

Adding a new react/typescript extension example to our repo. Merge Explore Query is an extension that provides a simple UI for Looker users to take an explore query url and merge it with another. Izzy had a external site setup for this purpose (https://www.merge.monster/), which is still used frequently by our customers and recommended to customers over chat or by our field teams however he'd like to stop maintaining and instead have something customers can leverage from the Looker org.