Improve handling of behindPreprocessorFlag argument in @Spyable attribute

[Matejkob]

This commit refines the implementation and diagnostic handling of the behindPreprocessorFlag argument for the @Spyable attribute. The key improvements include:

1. Enhanced Argument Extraction:

   • Updated the extraction logic for the behindPreprocessorFlag argument in the SpyableMacro.swift and Extractor.swift files. The new approach is more robust, ensuring the flag is accurately identified and processed within the macro expansion context.

2. Diagnostic Improvements:

   • Introduced a specific diagnostic message for cases where the behindPreprocessorFlag argument does not conform to the expected static string literal format. This change aims to provide immediate, actionable feedback to developers, aiding in quicker resolution of configuration errors.

3. Test Suite Expansion:

   • Enhanced the test suite to include scenarios testing the behindPreprocessorFlag argument's behavior. These tests validate both the successful extraction of the flag and the emission of diagnostics when the argument format is incorrect.

Fixes #84.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (4e093c3) 96.70% compared to head (622cdac) 98.36%.

Files	Patch %	Lines
...s/SpyableMacro/Diagnostics/SpyableDiagnostic.swift	66.66%	3 Missing ⚠️
.../SpyableMacro/Diagnostics/SpyableNoteMessage.swift	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
+ Coverage   96.70%   98.36%   +1.65%     
==========================================
  Files          17       18       +1     
  Lines         699      736      +37     
==========================================
+ Hits          676      724      +48     
+ Misses         23       12      -11     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.