test: migrate LeetCode 208 to new directory structure with tests

[kobukuro]

Migrate Trie (Prefix Tree) implementation from old directory structure to standardized problems/medium/ layout following project conventions.

Code improvements:

• Remove unnecessary is_end parameter from init
• Add type hints (dict[str, Trie]) for better clarity
• Simplify variable naming (curr_node → curr, w → c)
• Follow PEP 8 naming convention (startsWith → starts_with)

Documentation:

• Move complexity comments into proper docstrings
• Update README.md: increment total count (87→88) and medium count (25→26)
• Update docs/MEDIUM.md: add problem 208 entry

Test coverage:

• LeetCode official example test case