client/web: add confirmation dialogs

Add confirmation dialogs for disconnecting and stopping advertisement
of a subnet route.

Updates #10261

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

client/web/src/utils/util.ts

@@ -14,6 +14,13 @@ export function assertNever(a: never): never {
  */
 export function noop() {}
 
+/**
+ * isObject checks if a value is an object.
+ */
+export function isObject(val: unknown): val is object {
+  return Boolean(val && typeof val === "object" && val.constructor === Object)
+}
+
 /**
  * pluralize is a very simple function that returns either
  * the singular or plural form of a string based on the given