examples: show how to turn on CORS in a vweb server app

examples/vweb/cors/vweb_cors_example.v

@@ -0,0 +1,62 @@
+import time
+import vweb
+
+// See https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
+// and https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#preflighted_requests
+// >  Cross-Origin Resource Sharing (CORS) is an HTTP-header based mechanism that allows
+// >  a server to indicate any origins (domain, scheme, or port) other than its own from
+// >  which a browser should permit loading resources...
+
+// Usage: do `./v run examples/vweb/cors/` to start the app,
+// then check the headers in another shell:
+//
+// 1) `curl -vvv -X OPTIONS http://localhost:45678/time`
+// 2) `curl -vvv -X POST http://localhost:45678/
+
+struct App {
+	vweb.Context
+}
+
+// Browsers like Chrome use OPTIONS requests for their preflight requests.
+// This route method ensures, that they will get the headers they need, to allow CORS.
+@['/:path...'; options]
+pub fn (mut app App) preflight(path string) vweb.Result {
+	app.add_header('Access-Control-Allow-Methods', 'GET,HEAD,PUT,PATCH,POST,DELETE')
+	app.add_header('Vary', 'Access-Control-Request-Headers')
+	return app.text('ok')
+}
+
+// before_request will be called for each route. *Either* use the add_header here, to allow
+// *all* endpoints, or add it for each specific route, including for the preflight one above.
+// Note that browsers disallow using the Access-Control-Allow-Origin header more than once.
+pub fn (mut app App) before_request() {
+	app.add_header('Access-Control-Allow-Origin', '*')
+}
+
+// time is a simple POST request handler, that returns the current time. It should be available
+// to JS scripts, running on arbitrary other origins/domains.
+@[post]
+pub fn (mut app App) time() vweb.Result {
+	// *Either* use the .add_header line here, *or* the one in before_request, but *not both*.
+	// app.add_header('Access-Control-Allow-Origin', '*')
+	return app.json({
+		'time': time.now().format_ss_milli()
+	})
+}
+
+fn main() {
+	println("
+To test, if CORS works, copy this JS snippet, then go to for example https://stackoverflow.com/ , 
+press F12, then paste the snippet in the opened JS console. You should see the vweb server's time:
+	
+var xhr = new XMLHttpRequest();
+xhr.onload = function(data) {
+	console.log('xhr loaded');
+	console.log(xhr.response);
+};
+xhr.open('POST', 'http://localhost:45678/time');
+xhr.send();
+
+	")
+	vweb.run(&App{}, 45678)
+}