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?

In plain flavors (TypeScript and JavaScript), having scss files starting with _ are not ignored from the require.context causing build errors because they are treated as root stylesheets.

What is the new behavior?

Files starting with an _ are now properly ignored from the require.context() that maps out all the files in the app and will no longer cause build errors due to treating these stylesheets as root ones.

Fixes/Implements/Closes #[Issue Number].