Feat: configure recently successful delay

[RobertBoes]

This allows users to configure the recentlySuccessful delay, which currently is set to 2000 with no option to change this. The delay can be set using the second (or third) parameter of creating a form, this config could in the future be used to control other aspects of the form helper

Usage:

const form = useForm({
  name: 'Foo',
}, {
  recentlySuccessfulDelay: 5000,
})

// Or set it after form creation
form.recentlySuccessfulDelay(5000).post('/url')