Add helper functions to identify types of metrics / metric sets

[EmilHvitfeldt]

for use as is_class_metric() to determine if accuracy() is the right thing. We have an as_tibble() for metric_sets https://github.com/tidymodels/yardstick/blob/be744a3d419398a165aa5bbea0e7f14e750d5b79/R/aaa-metrics.R#L317 but it is not documented well enough