fix(webpack): respect hmr flag

[edusperoni]

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• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

webpack 5 includes nativescript-hot-loader and the hmr runtime regardless if it's being used or not (which have a nasty side effect of removing sourcemaps for code coverage)

What is the new behavior?

Those are only added when env.hmr is true