Use named extract for code content (#2263)

This is to avoid partial matching with option like `code_folding = FALSE`

Found in **distill** while it was warning for non empty chunk content while using
````markdown
```{r, child="mychild.Rmd"}
```
````

R/block.R

@@ -55,7 +55,7 @@ call_block = function(block) {
 
   if (opts_knit$get('progress')) print(block)
 
-  params$code = parse_chunk(params$code) # parse sub-chunk references
+  params[['code']] = parse_chunk(params[['code']]) # parse sub-chunk references
 
   ohooks = opts_hooks$get()
   for (opt in names(ohooks)) {

R/parser.R

@@ -109,7 +109,7 @@ parse_block = function(code, header, params.src, markdown_mode = out_format('mar
   code = parts$code
 
   label = params$label; .knitEnv$labels = c(.knitEnv$labels, label)
-  if (length(code) || length(params$file) || length(params$code)) {
+  if (length(code) || length(params[['file']]) || length(params[['code']])) {
     if (label %in% names(knit_code$get())) {
       if (identical(getOption('knitr.duplicate.label'), 'allow')) {
         params$label = label = unnamed_chunk(label)