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.
 }