Build a word from letters

Write a function canBuildWord(letters, word) that checks if you can make word from the letters of letters.

• Both arguments are strings of Latin letters. Either of them can be empty.
• Every character of letters can be used only once: 'apple' has one e, so it cannot make 'peel'.
• Case does not matter: 'A' and 'a' are the same letter.
• Letters that are left over are fine.
• An empty word can always be made.
• Return a boolean: true or false.

Tip: a counter in a Map tells you how many times each letter is still free.