feat: add output flag with Destination interface.

Added -o flag to write results to files using a new Destination interface.

Changes:
- Add Destination interface with StdoutDestination and FileDestination implementations.
- Add -o flag to CLI parser for output file specification.
- Update Config to use Destination instead of direct output handling.
- Refactor main to use Destination.Write() for result output.

internal/config/config.go

@@ -3,9 +3,10 @@ package config
 
 // Configuration settings for the program.
 type Config struct {
-	Source      Source // The source code given to the program.
-	Verbose     bool   // Whether or not to print debug logs.
-	Explanation bool   // Whether or not to print an explanation of the reduction.
-	Profile     string // If not nil, print a CPU profile during execution.
-	Statistics  bool   // Whether or not to print statistics.
+	Source      Source      // The source code given to the program.
+	Destination Destination // The destination for the final result.
+	Verbose     bool        // Whether or not to print debug logs.
+	Explanation bool        // Whether or not to print an explanation of the reduction.
+	Profile     string      // If not nil, print a CPU profile during execution.
+	Statistics  bool        // Whether or not to print statistics.
 }