feat: forbid working with Xcode below 9

[rosen-vladimirov]

Currently iOS Applications cannot be build with Xcode versions below 9. So forbid using such versions and show information to user that Xcode must be updated.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

When you have Xcode 8, you can try tns build ios and it will fail at some point with a native error.

What is the new behavior?

When you have Xcode 8, you can try tns build ios and it will fail with message that this Xcode is not supported and you have to upgrade it to build your app.

Fixes issue #3887