This function returns TRUE for temp-lighted vectors (class vlightr_templight)
or subclasses thereof, and returns returns FALSE otherwise.
Examples
# The following are `FALSE`
is_templight(12)
#> [1] FALSE
is_templight(highlight(12))
#> [1] FALSE
# The following are `TRUE`
is_templight(templight(12))
#> [1] TRUE
is_templight(templight_case(12))
#> Error in prepare_templight_functions(...): object 'highlighters_at' not found