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?

We're unable to use importHelpers because we rely on a custom __extends for extending native classes

What is the new behavior?

We override tslib in webpack so it always uses our custom @nativescript/core/tslib. This will allow importHelpers to work and we can completely drop "importHelpers": false support by NativeScript 9 or 10 while moving tslib to a devDependency