feat: add subscriptions

[sdanialraza]

Please describe the changes this PR makes and why it should be merged:
Adds subscriptions for mainlib

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR changes the library's interface (methods or parameters added)

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[OfficialSirH]

Me parece bien

Resolved.

[Jiralite]

According to the documentation¹ and the specification², ends_at may now be null. This means this logic needs to be modified as Date.parse(null) will result in NaN:

discord.js/packages/discord.js/src/structures/Entitlement.js

Lines 84 to 93 in f2f7f1f

	if ('ends_at' in data) {
	/**
	* The timestamp at which this entitlement is no longer valid
	* <info>This is only `null` for test entitlements</info>
	* @type {?number}
	*/
	this.endsTimestamp = Date.parse(data.ends_at);
	} else {
	this.endsTimestamp ??= null;
	}

I say this, but the entitlement object itself on the documentation does not have it marked as nullable. It probably needs updating (discord/discord-api-docs#7268).

Footnotes

1. https://discord.com/developers/docs/monetization/implementing-app-subscriptions#how-app-subscriptions-work ↩

2. https://github.com/discord/discord-api-spec/blob/c979cb269dbaf8cd09b8d0a39cc683d9623d8ce4/specs/openapi.json#L16491-L16495 ↩

Resolved.

Resolved.