Unify naming of `inclTax` and `exclTax`

[adevade]

Currently most methods and properties use variants of incTax and exTax, while one place (CartLine::$unitPriceInclTax) uses inclTax.

This could be unified to inclTax and exclTax in v2, which makes both variants equal in character length.

Discussed briefly in Discord 2025-11-20: https://discord.com/channels/497326121441034240/705350345857040464/1441115249792258160

[alecritson]

Would it be worth adding the new methods/properties and @deprecating the existing so that people can be better prepared 🤔

[adevade]

Sounds like a very good idea! 👍