insert method
Inserts the word
into the trie.
Implementation
void insert(String word, T value) {
var currentNode = _root;
for (final character in word.characters) {
currentNode = currentNode.putChildIfAbsent(character);
}
currentNode.isEndOfWord = true;
// The last character of the word stores the associated word value.
currentNode.value = value;
}