Add DuckDuckGo support and refactor caching system

Introduced DuckDuckGo as a new search provider alongside Google. Implemented a flexible caching system with in-memory, file-based, and no-op cache options to improve modularity. Updated dependencies and revised the project structure for improved maintainability.

pkg/cache/nop.go

@@ -0,0 +1,38 @@
+package cache
+
+import (
+	"io"
+)
+
+type Nop struct {
+}
+
+var _ Cache = Nop{}
+
+func (Nop) Get(_ string, _ io.Writer) error {
+	return ErrNotFound
+}
+
+func (Nop) GetString(_ string) (string, error) {
+	return "", ErrNotFound
+}
+
+func (Nop) GetJSON(_ string, _ interface{}) error {
+	return ErrNotFound
+}
+
+func (Nop) Set(_ string, _ io.Reader) error {
+	return nil
+}
+
+func (Nop) SetJSON(_ string, _ interface{}) error {
+	return nil
+}
+
+func (Nop) SetString(_ string, _ string) error {
+	return nil
+}
+
+func (Nop) Delete(_ string) error {
+	return nil
+}