Support RETURN keyword

[ulmer-a]

This PR adds support for the RETURN keyword in the parser and code generator. It includes an example for experimentation as well as parser and codegen tests.

[codecov]

Codecov Report

Merging #234 (2468cf3) into master (04bcbda) will increase coverage by 0.01%.
The diff coverage is 97.77%.

@@            Coverage Diff             @@
##           master     #234      +/-   ##
==========================================
+ Coverage   93.58%   93.60%   +0.01%     
==========================================
  Files          34       34              
  Lines        9173     9217      +44     
==========================================
+ Hits         8585     8628      +43     
- Misses        588      589       +1     

Impacted Files	Coverage Δ
src/ast.rs	87.67% <50.00%> (-0.16%)	⬇️
src/codegen/generators/pou_generator.rs	95.58% <100.00%> (+0.04%)	⬆️
src/codegen/generators/statement_generator.rs	99.78% <100.00%> (+<0.01%)	⬆️
src/parser/control_parser.rs	96.02% <100.00%> (+0.14%)	⬆️
tests/correctness/control_flow.rs	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 04bcbda...2468cf3. Read the comment docs.

[ghaith]

Could you also add a correctness test for an early return function?
Correctness tests will execute the code and return a value, not just codegen.