Every icon sourced from an installed icon library (as opposed to a local
icon_set() or a raw read_icon() file) has a file path of the form
<cache root>/<library>/<...name parts...>.svg (see 00_registry.R's
icon_fn$get()), which is exactly the library$sub$name expression used
to access it. icon_label() recovers that label straight from
icon_path() rather than requiring a name to be tracked separately —
so, unlike names(), it survives c()/rep()/subsetting on an
icon_vec (a vctrs rcrd, which cannot carry element names()).
icon_label(x)A character vector the same length as x giving each icon's
library$sub$name label. Icons not sourced from an installed library
(a local icon_set() or a read_icon() file outside the icon cache)
return NA.