feat(r): Add conversion from integer type to character()

[paleolimbot]

Closes #324.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (fd73986) 88.12% compared to head (0c0247c) 88.03%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
r/src/convert_array.c	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #345      +/-   ##
==========================================
- Coverage   88.12%   88.03%   -0.09%     
==========================================
  Files          72       68       -4     
  Lines       11579    11494      -85     
==========================================
- Hits        10204    10119      -85     
  Misses       1375     1375              

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