Update nuspec and build script to support `<license />` instead of the deprecated `<licenseUrl />`

* Add variable for dropdown-divider-margin-y

For us to reassign the nav-divider-margin-y variable here, we have to rearrange some of the variables.

Fixes #26660.

* bump bundlesize

With the current styles, it is not possible to nest a popover inside of
another popover if they have different placements because the arrow
styles of the parent popover will conflict with the arrow styles of the
child popover.