[R] String notation of nanoarrow types

From https://github.com/pola-rs/r-polars/pull/730#issuecomment-1906192408

Data types of the `arrow` package can be converted to string notation.

```r
> arrow::int16()$ToString()
[1] "int16"
```

Since it is easier to filter string vectors using the `%in%` operator, it would be great if the nanoarrow data type also had a string representation that matched the `arrow` data type.

Start with the nanoarrow data type conversion used by the R interface and compare its output with arrow::int16()$ToString(), which returns "int16". Done means nanoarrow data types expose string representations matching the arrow data type notation so they can be filtered with %in%.

Written by the indexing model from the issue text.