Review useForm types

[pedroborges]

After installing @types/lodash I got different type errors:

/inertia/packages/svelte/src/lib/useForm.ts:96:34
Error: No overload matches this call.
  Overload 1 of 4, '(target: {}, source: TForm): {} & TForm', gave the following error.
    Argument of type 'TForm | undefined' is not assignable to parameter of type '{}'.
      Type 'undefined' is not assignable to type '{}'.
  Overload 2 of 4, '(target: object, ...sources: any[]): any', gave the following error.
    Argument of type 'TForm | undefined' is not assignable to parameter of type 'object'.
      Type 'undefined' is not assignable to type 'object'. 
      if (typeof fieldOrFields === 'undefined') {
        defaults = Object.assign(defaults, cloneDeep(this.data()))



/inertia/packages/svelte/src/lib/useForm.ts:101:7
Error: Type '{ [x: string]: string | undefined; } | ({} & keyof TForm)' is not assignable to type 'TForm | undefined'.
  Type '{ [x: string]: string | undefined; }' is not assignable to type 'TForm'.
    '{ [x: string]: string | undefined; }' is assignable to the constraint of type 'TForm', but 'TForm' could be instantiated with a different subtype of constraint 'FormDataType'. 

      defaults = Object.assign(
        cloneDeep(defaults),


/inertia/packages/svelte/src/lib/useForm.ts:102:9
Error: No overload matches this call.
  Overload 1 of 4, '(target: {}, source: keyof TForm | { [x: string]: string | undefined; }): { [x: string]: string | undefined; } | ({} & keyof TForm)', gave the following error.
    Argument of type 'TForm | undefined' is not assignable to parameter of type '{}'.
      Type 'undefined' is not assignable to type '{}'.
  Overload 2 of 4, '(target: object, ...sources: any[]): any', gave the following error.
    Argument of type 'TForm | undefined' is not assignable to parameter of type 'object'.
      Type 'undefined' is not assignable to type 'object'. 
      defaults = Object.assign(
        cloneDeep(defaults),
        cloneDeep(typeof fieldOrFields === 'string' ? { [fieldOrFields]: maybeValue } : fieldOrFields),


/inertia/packages/svelte/src/lib/useForm.ts:109:13
Error: Type 'TForm | undefined' is not assignable to type 'TForm'.
  'TForm' could be instantiated with an arbitrary type which could be unrelated to 'TForm | undefined'. 
    reset(...fields) {
      const resolvedData: TForm = typeof data === 'object' ? cloneDeep(defaults) : cloneDeep(data!())
      const clonedData = cloneDeep(resolvedData) as TForm

This PR fixes those errors by added a check to convert the input data to an object if it's a function early in the useForm function.

I have also added other smaller type suggestions.