types: union value types on intersect and difference

[suneettipirneni]

Please describe the changes this PR makes and why it should be merged:

Closes #55

intersect and difference will provide a collection with a value type that is a union of the collection value type and value type of the collection to run against. Just like before, you need to have a matching key type.

For example:

Performing an intersection of Collection<string, string> onto a Collection<string, number> will yield the type Collection<string, number>;

Performing an difference of Collection<string, string> onto a Collection<string, number> will yield the type Collection<string, string | number>;

Performing an intersection or difference on a Collection<string, string> and a Collection<number, string> is an error since key types don't match.

Status and versioning classification:

• Code changes have been tested, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

[Lioness100]

I don't understand why it's K | T - shouldn't it just be T?

for (const [k, v] of other) {
	if (this.has(k)) coll.set(k, v);
}

The values in the new collection are coming straight from collection other (with value type T). The values of the original collection do not come into play at all.

[suneettipirneni]

I don't understand why it's K | T - shouldn't it just be T?

for (const [k, v] of other) {
	if (this.has(k)) coll.set(k, v);
}

The values in the new collection are coming straight from collection other (with value type T). The values of the original collection do not come into play at all.

This is true for intersect, I've gone ahead and fixed that. For difference it still uses values from both collections as seen here:

const coll = new this.constructor[Symbol.species]<K, V | T>();
for (const [k, v] of other) {
	if (!this.has(k)) coll.set(k, v);
}
for (const [k, v] of this) {
	if (!other.has(k)) coll.set(k, v);
}

[Lioness100]

Oh, sorry, I missed that 😅