feat: add De Bruijn indexed reduction engine

Add a new interpreter option (-i debruijn) that uses De Bruijn indices
for variable representation, eliminating the need for variable renaming
during substitution.

- Add -i flag to select interpreter (lambda or debruijn)
- Create debruijn package with Expression types (Variable with index,
  Abstraction without parameter, Application)
- Implement shift and substitute operations for De Bruijn indices
- Add conversion functions between lambda and De Bruijn representations
- Update CLI to support switching between interpreters
- Add De Bruijn tests to verify all samples pass

Closes #26

internal/config/config.go

@@ -1,6 +1,14 @@
 // Package "config" parses ad handles the user settings given to the program.
 package config
 
+// Interpreter specifies the reduction engine to use.
+type Interpreter string
+
+const (
+	LambdaInterpreter   Interpreter = "lambda"
+	DeBruijnInterpreter Interpreter = "debruijn"
+)
+
 // Configuration settings for the program.
 type Config struct {
 	Source      Source      // The source code given to the program.
@@ -9,4 +17,5 @@ type Config struct {
 	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.
+	Interpreter Interpreter // The interpreter engine to use.
 }

internal/config/parse_from_args.go

@@ -14,8 +14,20 @@ func FromArgs() (*Config, error) {
 	profile := flag.String("p", "", "CPU profiling. If an output file is defined, the program will profile its execution and dump its results into it.")
 	file := flag.String("f", "", "File. If set, read source from the specified file.")
 	output := flag.String("o", "", "Output. If set, write result to the specified file. Use '-' for stdout (default).")
+	interpreter := flag.String("i", "lambda", "Interpreter. The reduction engine to use: 'lambda' or 'debruijn'.")
 	flag.Parse()
 
+	// Validate interpreter flag.
+	var interpType Interpreter
+	switch *interpreter {
+	case "lambda":
+		interpType = LambdaInterpreter
+	case "debruijn":
+		interpType = DeBruijnInterpreter
+	default:
+		return nil, fmt.Errorf("invalid interpreter: %s (must be 'lambda' or 'debruijn')", *interpreter)
+	}
+
 	// Parse source type.
 	var source Source
 	if *file != "" {
@@ -52,5 +64,6 @@ func FromArgs() (*Config, error) {
 		Explanation: *explanation,
 		Profile:     *profile,
 		Statistics:  *statistics,
+		Interpreter: interpType,
 	}, nil
 }