fix: cache/parallel should be outside uglifyOptions

[DickSmith]

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: https://github.com/NativeScript/nativescript-dev-webpack/blob/master/CONTRIBUTING.md#testing-locally-by-running-e2e-tests
• Tests for the changes are included.

What is the current behavior?

cache/parallel are inside uglifyOptions.

What is the new behavior?

cache/parallel are outside uglifyOptions per the documentation.
https://webpack.js.org/plugins/uglifyjs-webpack-plugin/#cache
https://webpack.js.org/plugins/uglifyjs-webpack-plugin/#parallel
https://webpack.js.org/plugins/uglifyjs-webpack-plugin/#uglifyoptions

BREAKING CHANGES:
None.

Migration steps:
Existing users will need to update their configs.

[vchimev]

test

[vchimev]

Thanks a lot, @DickSmith! 👍