Add support for named external

[SirLynix]

Thanks to @NiiRoZz for the initial work.

I updated it to make it more consistent and avoid some hacks like adding a prefix to VariableValueExpression.

[codecov]

Codecov Report

Attention: Patch coverage is 88.00000% with 15 lines in your changes missing coverage. Please review.

Project coverage is 78.05%. Comparing base (ff3c31b) to head (4223342).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/NZSL/GlslWriter.cpp	68.00%	8 Missing ⚠️
src/NZSL/Ast/SanitizeVisitor.cpp	90.76%	6 Missing ⚠️
include/NZSL/Ast/Compare.inl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
+ Coverage   76.57%   78.05%   +1.48%     
==========================================
  Files         158      158              
  Lines       21114    22773    +1659     
  Branches     6469     7759    +1290     
==========================================
+ Hits        16167    17776    +1609     
- Misses       4858     4900      +42     
- Partials       89       97       +8     

Files with missing lines	Coverage Δ
include/NZSL/Ast/Nodes.hpp	45.45% <ø> (ø)
include/NZSL/Ast/SanitizeVisitor.hpp	100.00% <ø> (ø)
src/NZSL/Ast/Cloner.cpp	88.39% <100.00%> (+0.02%)	⬆️
src/NZSL/Ast/ReflectVisitor.cpp	93.06% <ø> (ø)
src/NZSL/LangWriter.cpp	89.90% <100.00%> (+0.08%)	⬆️
src/NZSL/Parser.cpp	88.99% <100.00%> (+0.01%)	⬆️
src/NZSL/SpirvWriter.cpp	85.50% <100.00%> (ø)
tests/src/Tests/ErrorsTests.cpp	100.00% <ø> (ø)
tests/src/Tests/ExternalTests.cpp	100.00% <100.00%> (ø)
include/NZSL/Ast/Compare.inl	68.18% <50.00%> (-0.07%)	⬇️
... and 2 more

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