Implement EXIT and CONTINUE loop control statements

[ulmer-a]

This PR adds support for the EXIT and CONTINUE keywords in while and for loops and adds parser, codegen and correctness tests.

This will close #231 and close #236

[codecov]

Codecov Report

Merging #241 (e12fc45) into master (2966359) will increase coverage by 0.07%.
The diff coverage is 98.25%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
+ Coverage   95.03%   95.10%   +0.07%     
==========================================
  Files          35       35              
  Lines        9763     9987     +224     
==========================================
+ Hits         9278     9498     +220     
- Misses        485      489       +4     

Impacted Files	Coverage Δ
src/ast.rs	90.66% <33.33%> (-0.67%)	⬇️
src/codegen/generators/statement_generator.rs	99.80% <100.00%> (+0.02%)	⬆️
src/parser/control_parser.rs	96.15% <100.00%> (+0.36%)	⬆️
tests/correctness/control_flow.rs	100.00% <100.00%> (ø)

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[ghaith]

Looks good, but I think we could benefit from the following tests:

• Codegen tests where the exit/continue is inside an IF nested in a loop
• Correctness tests where the exit/continue is inside an IF nested in a loop (Should exit the loop)
• Correctness test with two nested loops and an exit in the inner loop (Should not exit the outer loop)

[ulmer-a]

Should be covered now!