Add CachedValue and CachedFunction

[fregante]

First pass for:

• Add a get/set helpers to the function cache.function returns #40

This new API is supposed to replace the old "Map-like" API and merge it with cache.function.

Simple usage

const choice = new CacheItem<boolean>('choice', {
	maxAge: {days: 30}
});
if (await choice.get() === undefined) {
	await choice.set(confirm('Want some cookies?'))
}

resetCacheButton.addEventListener('click', async () => {
	await choice.delete()
	location.reload()
})

Usage with updater

const email = new CacheItem<boolean>('choice', {
	maxAge: {days: 30},
	updater: async (user: string) => {
		const response = fetch('/api?user=' user);
		return response.text();
	}
});


const user1 = await email.get('koalaman');
const user2 = await email.get('supercat');

if (!await email.getCached('ironbird')) {
	alert('User `ironbird` not in cache')
}

refreshCacheButton.addEventListener('click', async () => {
	await email.getFresh('supercat')
})

Next steps

• Port old tests to new API
• Test new APIs (like getCached)
• Write type tests for new API with vitest
• Inline old logic in cache-item instead of importing it. Not strictly necessary at this point
• Move old logic to ./legacy.ts and new to ./index.ts
• Update readme
• Write migration guide + how to use the legacy API

[fregante]

Typing this is a supreme PITA, so for the sake of types and documentation, I'll probably export two classes:

• CacheItem
• DynamicCacheItem (or UpdatableCacheItem)