[6.x] Fix error with `{{ sc:cart:raw_tax_total_split }}` tag

This pull request fixes an error when using the {{ sc:cart:raw_tax_total_split }} tag.

Previously, the raw_tax_total_split tag was getting the line items from the tag's ->items() method which meant it was getting the line items post-augmentation.

This would then cause an issue when trying to sum the tax totals because the tax amounts would be augmented as strings (eg. £12.50) instead of integers.

This refactors that implementation so we're dealing with the LineItem methods, rather than augmented arrays.

duncanmcclean / simple-commerce

[6.x] Fix error with `{{ sc:cart:raw_tax_total_split }}` tag #1037