The Name Analyzer crunches names with the full string toolbox.
Implement:
1. static String initials(String first, String last) — both initials
upper-cased with a dot between: initials("ada","lovelace") is "A.L.".
Null or blank parts are skipped entirely (initials(null,"hopper") is
"H."; both blank → "").
2. static int vowelCount(String text) — count a e i o u in any case.
3. static String maskEmail(String email) — replace everything before and
including the @ with "***@"; input without @ comes back unchanged.
maskEmail("ada@example.com") is "***@example.com".
4. static boolean isUsername(String s) — a valid username is 3–16
characters, letters/digits/underscore only, and does NOT start with a
digit. Null is invalid.
Difficulty: beginner