[bugfix] OKL for loop init statement

[kris-rowe]

Description

Closes #610

• Ensures that an OKL for loop counter variable is initialized.
• Provides descriptive error messages related to OKL for loop init statements

[codecov]

Codecov Report

Merging #618 (2178381) into main (7df5b36) will decrease coverage by 0.69%.
The diff coverage is 100.00%.

❗ Current head 2178381 differs from pull request most recent head 8d4193c. Consider uploading reports for the commit 8d4193c to get more accurate results

@@            Coverage Diff             @@
##             main     #618      +/-   ##
==========================================
- Coverage   77.26%   76.56%   -0.70%     
==========================================
  Files         262      265       +3     
  Lines       19567    19514      -53     
==========================================
- Hits        15118    14941     -177     
- Misses       4449     4573     +124     

Impacted Files	Coverage Δ
src/occa/internal/lang/modes/oklForStatement.cpp	91.79% <100.00%> (+0.55%)	⬆️
src/occa/internal/lang/variable.cpp	71.25% <100.00%> (+2.39%)	⬆️
src/occa/internal/core/launchedKernel.cpp	0.00% <0.00%> (-100.00%)	⬇️
src/occa/internal/core/launchedDevice.cpp	0.00% <0.00%> (-79.67%)	⬇️
src/occa/internal/lang/expr/callNode.cpp	73.68% <0.00%> (-9.22%)	⬇️
src/dtype/builtins.cpp	41.66% <0.00%> (-5.56%)	⬇️
src/occa/internal/utils/string.cpp	81.33% <0.00%> (-4.00%)	⬇️
src/occa/internal/utils/styling.cpp	93.45% <0.00%> (-3.74%)	⬇️
src/occa/internal/modes/serial/registration.cpp	91.11% <0.00%> (-2.23%)	⬇️
src/core/kernelArg.cpp	79.80% <0.00%> (-1.93%)	⬇️
... and 25 more