feat: add file input flag to CLI.

Added -f flag to allow reading lambda expressions from files.

Changes:
- Add FileSource type to read from file paths.
- Add -f flag to command-line parser.
- Implement validation to prevent conflicting -f and positional arguments.

internal/config/source.go

@@ -27,3 +27,15 @@ func (s StdinSource) Extract() (string, error) {
 
 	return string(data), nil
 }
+
+// A source reading from a file.
+type FileSource struct{ Path string }
+
+func (s FileSource) Extract() (string, error) {
+	data, err := os.ReadFile(s.Path)
+	if err != nil {
+		return "", err
+	}
+
+	return string(data), nil
+}