fix: deep partial .value() and .set(...) types for forms

[sillvva]

fixes #14835

The form's default values are an empty object {} when defaults are not provided by .set(). Therefore, the .value() getter should return a deep partial of the schema type as all values are initially undefined.

Plus, deep partial .set() allows setting some, but not all values initially.

DeepPartial Demo

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14837-svelte.vercel.app/

[dummdidumm]

Thank you! For value() this makes sense, for set(...) it's debateable because it may be better to require you to provide everything that's, well, required.

[sillvva]

You're not really validating on .set(), though. Sometimes forms only have one initial value set. And having to provide the entire form, including a bunch of empty strings/undefined is more cumbersome.

If the user tries to submit the form with the initial values, it would make sense the validation would fail.