[knitr engine] Use `prql` as code block name

[eitsupi]

Currently, the code block for prql is converted to elm.
This is because prql is not considered a valid language by Pandoc.

https://github.com/eitsupi/prqlr/blob/61b207365f0101326ebdbe70d2e364a33a1cf2dd/R/knitr-engine.R#L21-L26

When a prql is registered in Pandoc (and the upstream KDE syntax highlighting), these lines can be deleted.

https://github.com/jgm/skylighting#adding-new-syntaxes
https://github.com/KDE/syntax-highlighting

[eitsupi]

Upstream issue PRQL/prql#2213