Simplify `str_arg_char` template specialization

[rmisev]

• One string class parameter is used instead of the parameter pack
• Simplified use of the str_arg constructor with string pointer and length parameters - length can now be any integer.
• Use class template argument deduction with str_arg in test-str_arg.cpp.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.66%. Comparing base (e2ec706) to head (066d985). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #77 +/- ## ========================================== - Coverage 99.67% 99.66% -0.01% ========================================== Files 14 14 Lines 2427 2422 -5 Branches 382 382 ========================================== - Hits 2419 2414 -5 Misses 8 8 ```

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