yearqtr() could optionally output 2025Q4 format

```r
data.table::yearqtr("2025-12-31")
#[1] 2025.75
data.table::yearqtr(as.Date("2025-12-31"))
#[1] 2025.75
```
there could be new argument `format = c("numeric", "character")` and the latter one would produce
```
2025Q1
2025Q2
2025Q3
2025Q4
```
so commonly used format

current workaround is to have wrapper
```
paste(data.table::year("2025-12-31"), data.table::quarter("2025-12-31"), sep = "Q")
```