Add support for non-dictionary OpenFOAM files

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.50%. Comparing base (dc4a9ea) to head (dd3a1e8).

Files	Patch %	Lines
foamlib/_files/_serialization.py	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #125 +/- ## ========================================== - Coverage 89.60% 89.50% -0.10% ========================================== Files 9 9 Lines 808 810 +2 ========================================== + Hits 724 725 +1 - Misses 84 85 +1 ```

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