[Form] Support resetting a form to initial values

[mj12albert]

Feature request

Summary

The Form component supports a native submit button - <button type="submit"> - but not a reset button:

<Form onSubmit={() => console.log('submitted')}>
   <Field.Root>
     {/*  a normal Field.Control that renders an input */}
   </Field.Root>

   {/* works */}
   <button type="submit">
     Submit
   </button>

   {/* doesn't work */}
   <button type="reset">
     Reset
   </button>
</Form>

Examples in other libraries

• React Aria provides a reset button <Form><Button type="reset" /></Form>
• React Hook Form supports a native reset button and also a reset() method via a hook

Motivation

It's often useful to be able to discard all edits in a form and reset it to the initial values. This will could require Fields to additionally track the initialValue.