Checks if provided object is a string i.e. a length-one character vector.
is.string(x)a logical value indicating whether provided object is a string
is.string("foobar") # [1] TRUE
#> [1] TRUE
is.string(1) # [1] FALSE
#> [1] FALSE
is.string(c("foo", "bar")) # [1] FALSE
#> [1] FALSE