Number scale helper function

[ximinez]

High Level Overview of Change

After looking again at #6246 changes, I realized there could be a simpler function call.

Context of Change

• Add canonical "scale" computation to Number #6246

Type of Change

• Refactor (non-breaking change that only restructures code)

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.9%. Comparing base (a6b7d4e) to head (c90a3f9).

Files with missing lines	Patch %	Lines
src/xrpld/app/tx/detail/InvariantCheck.cpp	50.0%	6 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                        @@
##           tapanito/lending-vault-invariant   #6384   +/-   ##
================================================================
  Coverage                              79.9%   79.9%           
================================================================
  Files                                   842     842           
  Lines                                 65625   65627    +2     
  Branches                               7253    7253           
================================================================
+ Hits                                  52457   52459    +2     
  Misses                                13168   13168           

Files with missing lines	Coverage Δ
include/xrpl/protocol/STAmount.h	95.6% <100.0%> (+0.1%)	⬆️
src/xrpld/app/misc/LendingHelpers.h	94.7% <100.0%> (ø)
...rc/xrpld/app/tx/detail/LoanBrokerCoverWithdraw.cpp	96.3% <100.0%> (ø)
src/xrpld/app/tx/detail/InvariantCheck.cpp	92.8% <50.0%> (ø)

... and 3 files with indirect coverage changes

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