Add get_log_level API to query current logging level

[krystophny]

Summary
Add a small public API to query the current logging level from Fortran: get_log_level(). This complements set_log_level() and is useful for applications/tests to save/restore verbosity and to assert logger state.

Proposed changes

• Add integer function get_log_level() to module fortplot_logging, returning the current log level.
• Re-export it from the top-level fortplot module alongside set_log_level and LOG_LEVEL_* constants.
• Add a focused FPM test that exercises transitions and verifies that get_log_level() matches expectations.

Rationale

• Symmetric API (setter + getter) improves usability.
• Enables tests to assert logger state without relying on output capture.
• Zero behavior change unless API is called; no impact to existing code.

Verification

• fpm test should pass, including the new test.
• No output artifacts affected.