Adding `%sh` support + knitr engine shorthand

[zacdav-db]

Can now use %sh magic in rmarkdown:

This is achieved thanks to %%sh in a python context.

    ```{databricks_sh}
    ls -la /

Additionally can use new `db_{r/py/scala/sh}` shorthand
```r
    ```{db_r}
    library(sparklyr)
    sc <- spark_connect(method = "databricks")