Add a feature to enable tax on shipping boolean inside the TaxZone model

[cambad]

Context

Please describe your Pull Request as best as possible, explaining what it provides and why it is of value, referencing any related Issues.

I implemented an e-commerce website for a client that has mainly customers in the USA. They wanted to have the option to have tax on the shipping amount or not. So I decided to first extend the SystemTaxDriver of Lunar and add the code inside of it (and all the features).

Now, that the site is in production and the feature is working well, I wanted to share this option as I think it could be interesting for other people.

I tested the implementation with the Lunar's demo and inside my project too, to be sure everything is working well.

If you need more information, don't hesitate to ask! Please note that as it's a new feature I targeted the main branch.

Documentation updates

I added the tax_on_shipping field inside the TaxZone part of the documentation.

Automated tests

I wrote new tests relative to the feature inside the SystemTaxDriverTest class.

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Name	Status	Preview	Comments	Updated (UTC)
lunar-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 6, 2024 9:35pm

[glennjacobs]

Closing, as I don't believe this is required, as we have tax classes applied to shipping options that allow you to set a zero rate.