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.
• You have signed the CLA.
• All existing tests are passing:
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/WritingUnitTests.md.

What is the current behavior?

Webpack 1.50 (1.40?) now pre-parses CSS/SCSS using the 'css-loader' and includes the AST for the CSS instead of the raw CSS. This now causes breaking issues when you use addTaggedAdditionalCSS as the file that is webpacked is now no longer CSS, but an AST tree.
All apps that used require(css/scss) with addTaggedAdditionalCSS are now broken.

What is the new behavior?

This changes the addTaggedAdditionalCSS to actual run the same detection code that is used in the fromURI, and abstracts that code into a new function fromDetect Now both fromURI and addTaggedAdditionalCSS call fromDetect so they share the same logic to detect which way the css was handled.

This PR FIXES a BREAKING change caused by a change in the Webpack configuration...