feat(ios-dark-mode): adapt dynamic system colors - #7826

This pull request depends on #7817 to build successfully.

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.
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• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/DevelopmentWorkflow.md#running-unit-tests.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/WritingUnitTests.md.

What is the new behavior?

This pull request adapts (dynamic) system colors from UIKit for:

• window background color,
• view controller background color,
• activity indicator style,
• tabs colors,
• text-view hint and text colors.

Implements a part of #7800.