Capitalises the first character and lowercases the rest of each element.
Examples
x <- generate_string(nchar = 10, vector = LETTERS, vector_length = 10)
str_proper(x)
#> [1] "Efuwyzewux" "Jmoyzmqmde" "Zvyttlthhs" "Zjahtvkrwt" "Jytzolazuz" "Clcrocykmw" "Mujdyywtse" "Aznlljxpbg" "Qzhalzuqyt" "Bbkmmvuyqd"