feat: Add dedupe functions

[IAmJSD]

This adds the following functions (they behave like cache in React and across sub-requests using the framework fetch function):

• dedupe - Creates a deduplicated function. This means that within a request, if multiple calls are made with the same arguments, the underlying function will only be called once and the result will be cached and returned for all subsequent calls.
• getUnderlyingDedupeFunction - Gets the underlying function that was turned into a proxy.

There is also DedupeCache to purge function calls from the cache. It uses the name dedupe since whilst the functionality is well received in React server components, the name has caused some confusion. I can't remember the origin of this name, but I think it was one of Theo's streams (?), and its the name I personally like the best.

Right now, if you have a function that is used in multiple places in Svelte, there is a lot of complexity in getting type safe shared values that can work across multiple requests. For example, there are hooks, but these require pre-knowledge of how you would like to execute your actions data flow, separating the data fetching layer from the data handling itself. Additionally, this locals object gets killed off when you make a sub-request using the internal fetch client. But what if you want to persist your cache for the whole web request lifecycle?

Say for example I have a getUserByToken function. The contents of this function are not super important, but it takes in a token and returns a user.

export const getUserbyToken = async (token: string) => {
    // TODO: Whatever here
};

So now I have a function I can share across my codebases API, pages, and layouts. This is a useful abstraction, but there are a few ways this can become problematic:

• What if I want to quickly use a function from my layout and page? I can use hooks, but this makes this much more complex, especially if its a function type where I might want multiple calls with different arguments (which this handles).
• What if this is a sub-request? The locals are not persisted, so I have to make multiple calls even though I already fetched this data.

The dedupe function fixes all of this by creating a per-request function cache and then shares said cache with sub-requests. Using this function is simple, you simply wrap the function:

export const getUserbyToken = dedupe(async (token: string) => {
    // TODO: Whatever here
});

The type of the function will be identical and it will be functionally identical to use, except if called with the same arguments in a (sub-)request, it will return the same results. This allows a simple abstraction for a issue that is complex to solve right now in SvelteKit.

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