Add formatter for a11y string

[schlagelk]

Checklist

• All new and existing tests pass
• I've read the Contribution Guidelines
• I've read Change Limitations

Motivation and Context

Milliseconds do not to really seem be supported in any of the date formatting or NSCalendar APIs. Because of that, there is issue #19.

Description

This PR adds a fractional seconds calculation to TimeInterval and a string formatter to DateComponentsFormatter that will append any available fractional seconds as milliseconds (it also adds a localizable string for "milliseconds"). This PR includes s a unit test to verify said code.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[schlagelk]

I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[taquitos]

Awesome, thank you!

[garrettmurray]

We're currently discussing this path because of a few issues:

1. In an ideal world, we'd use our existing DateComponentsFormatter to simply use milliseconds since it does the localization for us including locale ordering, etc. But obviously DCF doesn't support anything smaller than seconds.
2. The issue with appending this formatted string onto the end of the existing a11y string is that, in some locales, this might not be correct. We're not entirely sure, but given the history of how locales work it's entirely possible there's a locale in which milliseconds come first, perhaps.
3. We don't have a good solution to present instead 😅

We'll discuss a bit more internally and circle back to you on this. Greatly appreciate your PR and your patience. Thanks!