Add `Spree::ZERO` constant

[mamhoff]

Summary

Ruby's BigDecimal library does not come with a constant that denotes zero. We don't need to be creating a new zero every time we need one. Also, there's even a Rubocop that tells people to use an integer for integer-type BigDecimal instantiations, but a string for float-type BigDecimals (See https://github.com/rubocop/rubocop-performance/blob/master/lib/rubocop/cop/performance/big_decimal_with_numeric_argument.rb)

This all to say: Let's have a constant in the codebase that is zero. We can just reference it, we don't need so many zeros.

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.97%. Comparing base (db99cba) to head (d9e5c92). Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6291      +/-   ##
==========================================
+ Coverage   88.95%   88.97%   +0.01%     
==========================================
  Files         860      861       +1     
  Lines       18422    18416       -6     
==========================================
- Hits        16387    16385       -2     
+ Misses       2035     2031       -4     

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