Arcade: Add override for IsShipping condition in Localization.targets

[rladuca]

Right now, localization.targets requires IsShipping to be set in order to import the XLIFF tasks.

I don't think these should be coupled. Specifically, the WPF repository isn't marked as shipping as it creates transport packages that are not directly published.

We'd like to be able to override the IsShipping condition on the inclusion of XliffTasks.

[tmat]

WPF repository isn't marked as shipping as it creates transport packages that are not directly published

WPF should set IsShipping to true for libraries that eventually make it to customers. It does not matter if they are published directly to NuGet, thru an installer, or indirectly shipped by other component.

[nguerrera]

This sentence in the documentation is misleading :

https://github.com/dotnet/arcade/blame/fa19e5a7dd74bca0dbea83b8a306457483534eec/Documentation/RepoToolset/MigrationToArcade.md#L16

[tmat]

It's clarified in the next sentence:

• projects that produce NuGet packages that are not shipping on NuGet.org or other official channel (like part of an official installer),

I'll try to reword here to make it better:
https://github.com/dotnet/arcade/blob/master/Documentation/ArcadeSdk.md#isshipping-bool