Add a getOrSet() convenience method to delegate memoization / lazy initialization to the cache

[0x24a537r9]

When using this package for memoization / lazy initialization, there's a pretty common pattern for getting while also setting only if a value is not already set:

function getValue(key) {
  const cachedValue = cache.get(key);
  if (cachedValue !== undefined) return cachedValue; // Should this actually be using .has(key)? That would then require another .get() call if so...

  const newValue = someExpensiveFunction();
  cache.set(key, newValue);
  return newValue;
}

It seems to me that it's common enough (and subtle enough when it comes to checking null vs. undefined vs. using .has()) that it would be worthwhile to define a function like getOrSet:

function getValue(key) {
  return cache.getOrSet(key, () => expensiveFunction());
}

Apart from the convenience factor, this would put the burden of correctness when it comes to existence checking (which I suspect might actually incorrect above in the case of cached values of undefined) on this library's maintainers (who are much more qualified than the average user of this this package to know the safest behavior), and also might open an opportunity for slight optimization if the correct behavior is .has(), since without access to the internal state of the cache one would have to also add a .get() after the .has(), adding more function invocations and another lookup.

If this were deemed something worthwhile to add, the question is whether to have it be an independent function or an option in the options for get. E.g. it could be a new getOrSet or getWithDefault or memoize method, or a getDefault: () => T option for get. I think a new method avoids a) potentially dangerously unexpected/misunderstood behavior where it ends up performing a write operation when .get() sounds like it would only be a read operation, and B) it being buried in options documentation, leading to less use.