add lesson 1 - hello world
add lesson 2 - values
add lesson 3 - variables
add lesson 4 - constants
add lesson 5 - for
add lesson 6 - if/else
add lesson 7 - switch
add lesson 8 - arrays
add lesson 9 - slices
add lesson 10 - maps
add lesson 11 - range
add lesson 12 - functions
add lesson 13 - multiple return values
add lesson 14 - variadic functions
add lesson 15 - closures
add lesson 16 - recursion
add lesson 17 - pointers
add lesson 18 - structs
add lesson 19 - methods
add lesson 20 - interfaces