js: add experimental interface for integrating rclone into browsers

This works by compiling rclone to wasm and exporting the RC api to
javascript.

fs/rc/js/serve.go

@@ -0,0 +1,19 @@
+//+build none
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"mime"
+	"net/http"
+)
+
+func main() {
+	mime.AddExtensionType(".wasm", "application/wasm")
+	mime.AddExtensionType(".js", "application/javascript")
+	mux := http.NewServeMux()
+	mux.Handle("/", http.FileServer(http.Dir(".")))
+	fmt.Printf("Serving on http://localhost:3000/\n")
+	log.Fatal(http.ListenAndServe(":3000", mux))
+}