Add indicators of significant change after SiZeR

## Arguments

- object
an R object. Currently supported methods are for classes

`"derivatives"`

.- type
character;

`"change"`

adds a single variable to`object`

indicating where the credible interval on the derivative excludes 0.`"sizer"`

adds two variables indicating whether the derivative is postive or negative.- ...
arguments passed to other methods

- derivatives
an object of class

`"derivatives"`

, resulting from a call to`derivatives()`

.

## Examples

```
load_mgcv()
df <- data_sim("eg1", n = 400, dist = "normal", scale = 2, seed = 42)
m <- gam(y ~ s(x0) + s(x1) + s(x2) + s(x3), data = df, method = "REML")
## first derivatives of all smooths using central finite differences
d <- derivatives(m, type = "central") |>
add_sizer()
# default adds a .change column
names(d)
#> [1] ".smooth" ".by" ".fs" ".derivative" ".se"
#> [6] ".crit" ".lower_ci" ".upper_ci" ".change" "x0"
#> [11] "x1" "x2" "x3"
```