Add summary.yearmonth and summary.yearquarter

[robjhyndman]

This should work, for example:

summary(tsibble::tourism)
#> Registered S3 method overwritten by 'tsibble':
#>   method               from 
#>   as_tibble.grouped_df dplyr
#> Error in `FUN()`:
#> ! `summary.yearquarter()` not implemented.

^{Created on 2025-05-30 with reprex v2.1.1}