[SPARK-20438][R] SparkR wrappers for split and repeat

## What changes were proposed in this pull request?

Add wrappers for `o.a.s.sql.functions`:

- `split` as `split_string`
- `repeat` as `repeat_string`

## How was this patch tested?

Existing tests, additional unit tests, `check-cran.sh`

Author: zero323 <zero323@users.noreply.github.com>

Closes #17729 from zero323/SPARK-20438.