feat(core): Added css class for styling dialog nested elements - #10605

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/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/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

Ever since #10201, Dialog buttons, labels, and inputs are affected by global scope CSS. That brings the drawback of not being able to style the dialog specifically. One good example example, some apps ended up with light-colored text not being distinct in light-mode Dialogs because of how these apps were styled in global scope.

What is the new behavior?

Added ns-dialog-item css class to style nested dialog views.

Sample:

Label {
  color: #fff;
}

Label.ns-dialog-item {
  color: #000;
}