Fixes #470. Support legalLabelInsets on Apple Maps

[scarlac]

Fixes #470. This will add support for legalLabelInsets prop on iOS with Apple Maps. Code is adapted from RN core.

[gilbox]

Thanks! We should probably avoid -> notation. Would you mind adding an example for this?

[scarlac]

@gilbox Yeah, let's keep it consistent. I've removed it.

[gilbox]

@scarlac will you be able to add an example?

[scarlac]

@gilbox Ah! I am assuming you mean updating the documentation. Yes, I'll get right on it tomorrow and update this PR :)

[gilbox]

@scarlac I was talking about adding an example in the example/examples dir. That's how I smoke-test PRs since we don't have specs.

[scarlac]

@gilbox I've added an example to demonstrate the usefulness of the legalLabelInsets. I think it's fairly simple while showing a very realistic scenario of when it could be of use. Personally, I'm using it in an advanced way but opted for simplicity in the example, for the sake of understanding.

Let me know if there's more you need before it's ready for a merging.

[gilbox]

lgtm but I'd like to get a second opinion re: the async op

[gilbox]

thanks @scarlac ( ◠‿◠ )

[scarlac]

My pleasure! 👍