[R] Map `format = "text` to `format = "csv"` in `write_dataset()`

### Describe the enhancement requested

In `write_dataset()`, we should map "text" to "csv" like we do with `open_dataset()`

``` r
library(arrow)
library(dplyr)

tf <- tempfile()
dir.create(tf)
# gives an error
write_dataset(mtcars, tf, format = "text")
#> Error in write_dataset(mtcars, tf, format = "text"): A delimiter must be given for a txt format.

write_dataset(mtcars, tf, format = "csv")

# this works
open_dataset(tf, format = "text")
#> FileSystemDataset with 1 csv file
#> mpg: double
#> cyl: int64
#> disp: double
#> hp: int64
#> drat: double
#> wt: double
#> qsec: double
#> vs: int64
#> am: int64
#> gear: int64
#> carb: int64
```

^{Created on 2023-10-11 with [reprex v2.0.2](https://reprex.tidyverse.org)}

### Component(s)

R