feat: implement item management handlers with corresponding tests

internal/handlers/addItemHandler.go

@@ -0,0 +1,36 @@
+package handlers
+
+import (
+	"slices"
+
+	"github.com/StevanFreeborn/term-tier/internal/state"
+	tea "github.com/charmbracelet/bubbletea"
+)
+
+type addItemHandler struct {
+	keys        []string
+	description string
+}
+
+func NewAddItemHandler() KeyMsgHandler {
+	return addItemHandler{
+		keys:        []string{"n"},
+		description: "Add new item",
+	}
+}
+
+func (h addItemHandler) KeysHandled() []string {
+	return h.keys
+}
+
+func (h addItemHandler) Description() string {
+	return h.description
+}
+
+func (h addItemHandler) Match(s state.State, msg tea.KeyMsg) bool {
+	return s.Mode() == state.NavigationMode && slices.Contains(h.keys, msg.String())
+}
+
+func (h addItemHandler) Handle(s state.State, msg tea.KeyMsg) tea.Cmd {
+	return s.AddItem()
+}