Issue 407 allow including full files as externals

[99NIMI]

source files can now be included with "--include", "-i"
functions from included source files can be called like normal functions and must not be declared as @external
included files will be parsed with LinkageType::External and therefore be ignored for codegen

[ghaith]

Looks good, i tried the linking locally and it works.
I just have a comment on the method signatures in lib.

[riederm]

kannst du bitte beim commit noch in der commit-msg etwas zusammenfassen, was die zentralen Änderungen waren

[codecov-commenter]

Codecov Report

Merging #433 (f813a62) into master (163a0c6) will increase coverage by 0.16%.
The diff coverage is 92.57%.

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
+ Coverage   93.11%   93.27%   +0.16%     
==========================================
  Files          31       32       +1     
  Lines       11834    11978     +144     
==========================================
+ Hits        11019    11173     +154     
+ Misses        815      805      -10     

Impacted Files	Coverage Δ
src/lib.rs	85.23% <85.00%> (-0.18%)	⬇️
src/tests/external_files.rs	98.03% <98.03%> (ø)
src/cli.rs	97.62% <100.00%> (+0.17%)	⬆️
src/parser.rs	98.06% <100.00%> (ø)
src/test_utils.rs	100.00% <100.00%> (ø)
src/tests/multi_files.rs	100.00% <100.00%> (ø)
src/resolver.rs	96.62% <0.00%> (+0.08%)	⬆️
src/codegen/generators/expression_generator.rs	88.49% <0.00%> (+0.56%)	⬆️
src/validation.rs	90.64% <0.00%> (+0.98%)	⬆️
... and 2 more

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