fix(knitr): code compiles when eval = FALSE

[kbvernon]

Closes #547.

I think the original intent maybe was something like this (from the knitr-engine test):

Define variable `_x`:

```{extendr chunk_x, eval = FALSE}
let _x = 1;
```

Define variable `_y`:

```{extendr chunk_y, eval = FALSE}
let _y = 2;
```

Print:

```{extendr out, preamble = c("chunk_x", "chunk_y")}
rprintln!("x = {}, y = {}", _x, _y);
```

but this actually still works with a guard to return early when eval = FALSE because the preamble chunks are combined with the out chunk, compiled, and evaluated there.

Note: in knitr_engine.R, I took the liberty of cleaning up some of the code, e.g., using rlang::is_* functions and replacing lapply(options$preamble, knitr::knit_code$get) with knitr::knit_code$get(options$preamble) since it is already vectorized (or anyway it just subsets a list of code chunks with the character vector in preamble).