[V2] defaultValues of formOptions.looseSchema() should infer optional properties

[lucas-dtrn]

When using appFormOptions.looseSchema, the inferred type of defaultValues currently requires every property defined by the validation schema to be present.
For example:

import { appFormOptions } from '@tanstack/react-form';
import { z } from 'zod';

const formSchema = z.object({
  firstName: z.string(),
  lastName: z.string(),
});

appFormOptions.looseSchema({
  defaultValues: {
    firstName: undefined,
    lastName: undefined,
  },
  validators: [
    {
      triggers: ['change'],
      run: formSchema,
    },
  ],
});

The inferred defaultValues type is effectively:

type DefaultValues = {
  firstName: string | null | undefined;
  lastName: string | null | undefined;
};

Although each value may be undefined, both properties are still required. This forces users to explicitly assign undefined to every field.
For a loose schema, the properties themselves should instead be optional:

type ExpectedDefaultValues = {
  firstName?: string | null | undefined;
  lastName?: string | null | undefined;
};

This would allow partial or empty default values:

appFormOptions.looseSchema({
  defaultValues: {},
  validators: [
    {
      triggers: ['change'],
      run: formSchema,
    },
  ],
});

Expected behavior

looseSchema should infer optional properties for defaultValues, allowing fields to be omitted entirely.
Actual behavior
The properties are required, even though their values may be undefined, so all fields must be explicitly declared.

[LeCarbonator]

Sounds reasonable to me. Shouldn't have to be too tough to adjust, so expect this by the next alpha. I'll let you know in this discussion when it's out.