feat(index.d.ts): added type definitions

[pixelmund]

Sorry for reopening this again, was not happy because i didn't create a feature branch in my fork.

Those are the type definitions for svelte-forms-lib, there might be some optimizations but it's a good starting point.

Svelte currently does not support generics in components, there are some discussions around it but i couldn't find a solution.
For now we can use it like this:

import type {FormProps} from "svelte-forms-lib";

const formProps : FormProps = {
      initialValues: { name: "", email: "" },
      onSubmit: values => {
        /// values are inferred from initialValues
        alert(JSON.stringify(values));
      }
    };

fix #81

[tiaanduplessis]

Thanks @pixelmund ! LGTM

[larrybotha]

@pixelmund excellent, thanks for this!

[tjinauyeung]

🎉 This PR is included in version 1.7.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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