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?

main in nativescript.config.ts is ignored.

What is the new behavior?

main in nativescript.config.ts is read and used instead of package.json's main when set.

implements #9658

BREAKING CHANGES:

Possibly breaking if a project has a main field set in the nativescript.config.ts - since after this lands, this value will be used instead of package.json main.

The impact is likely very small, the steps to migrate:

1. (Option A) remove main from nativescript.config.ts if set
2. (Option B) update main to the correct path in nativescript.config.ts if set incorrectly