fix: improve fields type for generic components

[sillvva]

Follow up to #14893

This PR reintroduces the original solution I had proposed in #14893. A suggestion was made in that PR to simplify the type, but that appears to have been a mistake as it caused issues with generic components.

Included in this PR is a test that would otherwise fail - a function that represents a generic form component

function f10<
	Schema extends StandardSchemaV1<RemoteFormInput, unknown>,
	Form extends RemoteForm<StandardSchemaV1.InferInput<Schema>, unknown>
>(data: StandardSchemaV1.InferInput<Schema>, form: Form) {
	form.fields.set(data); // data is incompatible
	form.fields.allIssues(); // allIssues() does not exist
}
void f10;

After the update to the fields type in #14893, a type error occurs in generic <Form> components on the form.fields proxy. The .allIssues() method does not exist on the type and attempting to .set(data) yields a type error that data is not compatible despite being the same type as the form's input type.

Current workaround is to assign fields to a variable and assert it as RemoteFormFields<unknown>. But that reveals a separate issue, which I've proposed a fix for in #14973.

const fields = form.fields as RemoteFormFields<unknown>;

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Preview: https://svelte-dev-git-preview-kit-14974-svelte.vercel.app/

[sillvva]

I also created a type for better intellisense. The question is whether to export it.

Intellisense now shows:

const fields: RemoteFormFieldsRoot<StandardSchemaV1.InferInput<Schema>>

instead of:

const fields: IsAny<StandardSchemaV1.InferInput<Schema>> extends true ? RecursiveFormFields : StandardSchemaV1.InferInput<Schema> extends void ? {
    issues(): RemoteFormIssue[] | undefined;
    allIssues(): RemoteFormIssue[] | undefined;
} : RemoteFormFields<StandardSchemaV1.InferInput<Schema>>

[sillvva]

@dummdidumm when you get a chance, can you take a look at this one? You had previously approved #14893 and this is a followup to that one to fix generic components/functions.

Here's an example of a function that needs as any to ignore the incorrect type errors.

export function initForm<Input extends RemoteFormInput>(form: RemoteForm<Input, unknown>, getter: () => Input) {
	let hydrated = false;
	// eslint-disable-next-line @typescript-eslint/no-explicit-any
	form.fields.set(getter() as any);
	$effect(() => {
		const values = getter();
		if (!hydrated) return void (hydrated = true);
		// eslint-disable-next-line @typescript-eslint/no-explicit-any
		form.fields.set(values as any);
	});
}

#14973 was also discovered when testing workarounds for this.