Queries /collection. Returns the icons the API lists as visible: the
uncategorized icons plus every categorized icon. Aliases and hidden icons
exist and can be fetched with iconify_get(), but are not included.
Examples
head(iconify_icons("tabler"))
#> [1] "a-b" "a-b-2" "a-b-off" "abacus" "abacus-off"
#> [6] "abc"