feat: overdraft function

[ascandone]

Added an overdraft() function which returns the absolute value of the negative balance of the given account and currency; 0 if overdraft is non negative (see tests as reference)

This is a new numscript functionality which is not implemented in the current machine implementation; thus the functionality is under the experimental-overdraft-function feature flag

[codecov]

Codecov Report

Attention: Patch coverage is 82.89474% with 26 lines in your changes missing coverage. Please review.

Project coverage is 63.34%. Comparing base (661c6a1) to head (8f26655).

Files with missing lines	Patch %	Lines
internal/interpreter/interpreter.go	76.59%	7 Missing and 4 partials ⚠️
internal/cmd/run.go	0.00%	8 Missing ⚠️
internal/parser/antlr/numscript_parser.go	94.56%	5 Missing ⚠️
internal/interpreter/interpreter_error.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
+ Coverage   63.33%   63.34%   +0.01%     
==========================================
  Files          29       29              
  Lines        6395     6461      +66     
==========================================
+ Hits         4050     4093      +43     
- Misses       2167     2187      +20     
- Partials      178      181       +3     

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[ascandone]

EDIT: fixed

It turns out I couldn't call this function overdraft because that is already a token.
So we would need to use something like overdraft_amount instead (or workaround this issue, which I'm not sure is a good idea)