Fixes #474

[sappenin]

Fixes #474 by adding the isNegative function CurrencyAmount, and then properly supporting negative value detection in XRP and IssuedCurrency amounts.

This fix should also fix issue #527.

[codecov]

Codecov Report

Attention: Patch coverage is 93.44262% with 4 lines in your changes missing coverage. Please review.

Project coverage is 91.99%. Comparing base (2f07675) to head (d2c6596).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...org/xrpl/xrpl4j/codec/binary/types/AmountType.java	88.88%	0 Missing and 2 partials ⚠️
...a/org/xrpl/xrpl4j/model/transactions/Wrappers.java	95.12%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #535      +/-   ##
============================================
+ Coverage     91.92%   91.99%   +0.06%     
- Complexity     1823     1831       +8     
============================================
  Files           381      381              
  Lines          5040     5082      +42     
  Branches        417      431      +14     
============================================
+ Hits           4633     4675      +42     
+ Misses          273      272       -1     
- Partials        134      135       +1     

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