Add additional function overload for derived

[fielding]

Summary

Add an additional overload for derived to allow explicitly setting an initial value for non-async derived stores.

Details

After a long overdue svelte version update, I started getting some type errors on any non-async derived store that I had explicitly set an initial value. For example,

export const isSlow = derived(delta, ($delta) => $delta > 300, false);

Perhaps the correct way to handle the initial value is through the initial values of the stores that it is derived from, but I guess it feels safer to me to also explicitly set it on the derived store.

Assuming that this usage is acceptable then the additional overload keeps typescript happy. 😄

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[lukeed]

This is great, and much needed. Ran into it a number of times personally & I know it was a definition that Svelte used to include at some earlier point in the 3.x cycle.

Thanks!

[dummdidumm]

Is there a reason why this new signature isn't just the only signature? Are there reasons why this was split out?
(Agree with adding that signature)

[lukeed]

@dummdidumm This new signature has no $set usage. There's nothing wrong with having an overload pattern, and in derived's case, it is an overloaded function because there are different avenues for using/holding it.

[dummdidumm]

Silly me, didn't read that other definition closely enough. All good then!

[lukeed]

Thanks @fielding !

[Conduitry]

These types should be published in 3.38.0. Thanks!