Change name of the handler function to router

[ZooeyMiller]

From points raised from Founders and Coders students whilst doing this workshop (documented in foundersandcoders/master-reference#781) we would like to change the name of the handler/handler.js function/file to router/router.js.

The reasoning would be that this function is what handles the routing of the request (via the if(endpoint === "/") etc block). The logic for actually handling each request (serving resources etc) can be abstracted into separate functions, but the function directly passed to http.createServer always has to deal with routing.

The work needed for this would be to change all references to the handler function or the handler.js file to router or router.js respectively.

[ZooeyMiller]

@node-girls If this criteria is acceptable, could we add on a help-wanted and a 24PR label?

[claireinez]

Thanks @ZooeyMiller, we'd love for this to be done as part of 24 pull requests!