feat(svelte2tsx): autotype const load = ... declarations

[GauBen]

Hi!

This PR adds autotyping to const load = ... declarations, opening the door to SvelteKit "plugins"

Usage might be:

export const load = myPlugin((pluginData, event) => {
  return pluginData
})

// Dummy plugin
// We write the generics this way for the types to flow from output to input
function myPlugin<T>(
  load: T extends (event: infer Event) => infer Output
    ? (data: Record<string, unknown>, event: Event) => Output
    : never,
): T {
  return ((event) => load({ hello: "World!" }, event)) as T;
}

It works by rewriting const load = ... as const load = (...) satisfies PageLoad, letting TypeScript do the generic instantiation.

satisfies was added in TS 4.9 and peer TS is set to ^4.9.4, so this should not be a breaking change

[dummdidumm]

thank you!