feat: add Collection#ensure

[MidSpike]

This PR is a continuation of #40 which was closed by the author.

Inspired by Enmap#ensure, Collection#ensure gets an element with the specified key if it exists, otherwise sets it to the provided defaultValue and returns the defaultValue. Useful for things like per-guild settings where you want to get it or set it to a default value all in one call.

I found @kxmndz 's PR to be useful and wanted to ensure that it was maintained.

Potential Usage Example:

const guildConfigs = new Discord.Collection();
const defaultGuildConfig = {
  allowInvitesInChat: true,
};
const guildConfig = guildConfigs.ensure('566340180831633429', () => defaultGuildConfig);

Noticeable deviations from PR #40:

• The PR builds off of feat: add Collection#ensure #40 (rebased)
• The expected return value from collection#ensure can now be undefined|null whereas before a non-nullish return value was expected.
• 2 parameters have been added to the generator function key: K, collection: this.
  I believe that recycling these into the generator function will allow for less repetition in the end-user's code.
• @kyranet 's suggested edit was considered but directly interferes with the ability to have undefined|null as a valid ensured value, therefore it was not implemented into this PR.

Tests:
I'm not good at writing tests and would appreciate if someone help could contribute tests to cover the additional functionality added by this PR compared to the tests carried over by #40.

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
• This PR changes the library's interface (methods or parameters added)

[kyranet]

Just one last thing, let's keep the setup step tidy :P

[kyranet]

Now for real, one last thing before LGTM, sorry!

[kyranet]

One little documentation thing, then I'll re-approve.

[ImRodry]

Any ETA for when this will be merged? Would love to see this and #48 in a future release!