Group array of strings by first letter

// Write a function that takes an array of strings as argument

// Group those strings by their first letter

// Return an object that contains properties with keys representing first letters

// The values should be arrays of strings containing only the corresponding strings

// For example, the array ['Alf', 'Alice', 'Ben'] should be transformed to

// { a: ['Alf', 'Alice'], b: ['Ben']}