knitr engine improvements

[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

    ```{db_r}
    library(sparklyr)
    sc <- spark_connect(method = "databricks")
    ``` 

[codecov-commenter]

Codecov Report

Merging #36 (3042091) into main (3a8b9c1) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
- Coverage   11.47%   11.47%   -0.01%     
==========================================
  Files          23       23              
  Lines        2744     2746       +2     
==========================================
  Hits          315      315              
- Misses       2429     2431       +2     

Files Changed	Coverage Δ
R/knitr-engines.R	0.00% <0.00%> (ø)

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