Added NoteStore interface, mock and repository with Create method

internal/repository/notestore_test.go

@@ -5,11 +5,11 @@ import (
 )
 
 type mockNoteStore struct {
-	notes []models.Note
+	Notes []models.Note
 }
 
 func (m *mockNoteStore) SaveNote(note models.Note) (models.Note, error) {
-	note.ID = len(m.notes) + 1
-	m.notes = append(m.notes, note)
+	note.ID = len(m.Notes) + 1
+	m.Notes = append(m.Notes, note)
 	return note, nil
-}
+}