Fix embed and verbatim knitr's engine for revealjs (#4732)

* Do not override the knitr's `embed` engine with internal quarto's `embed` handler

Currently the `embed` handler is only for Jupyter.

Discovered while looking at #4712 - revealjs does not work with knitr embed engine

* Set echo = TRUE for embed and verbatim engine when revealjs

because those engine expect echo, but revealjs default to FALSE

* Add embed to ignored engine for chunk hook

* Add test for knitr revealjs fix regarding verbatim and embed

* Revert "Do not override the knitr's `embed` engine with internal quarto's `embed` handler"

This reverts commit 53d8703.

* Internal languages are only handlers of type cell

Other language are shortcodes

* For embed and verbatim engine, always set to TRUE

This will avoid format defaulting to echo: false to hide those chunks

Author: cderv

src/core/handlers/base.ts

@@ -279,8 +279,15 @@ function makeHandlerContext(
   return { context, results };
 }
 
+// return cell language handler only
 export function languages(): string[] {
-  return Object.keys(handlers);
+  const cellLanguage = [];
+  for (const [k, v] of Object.entries(handlers)) {
+    if (v.type === "cell") {
+      cellLanguage.push(k);
+    }
+  }
+  return cellLanguage;
 }
 
 export async function languageSchema(

src/resources/rmd/hooks.R

@@ -44,6 +44,11 @@ knitr_hooks <- function(format, resourceDir, handledLanguages) {
         options[["results"]] <- "hold"
       }
     }
+    # for source-only engine, always set `echo: TRUE`
+    if (options[["engine"]] %in% c("embed", "verbatim")) {
+      options[["echo"]] <- TRUE
+    }
+
     options
   }
 
@@ -162,9 +167,8 @@ knitr_hooks <- function(format, resourceDir, handledLanguages) {
       return(x)
     }
 
-    # verbatim and comment should do nothing
-    if (identical(options[["engine"]], "verbatim") ||
-        identical(options[["engine"]], "comment")) {
+    # verbatim-like and comment knitr's engine should do nothing
+    if (options[["engine"]] %in% c("verbatim", "embed", "comment")) {
       return(x)
     }
 

tests/docs/smoke-all/2023/03/09/code.R

@@ -0,0 +1 @@
+cat("this is code.R\n")

tests/docs/smoke-all/2023/03/09/revealjs-knitr-embed-verbatim.qmd

@@ -0,0 +1,28 @@
+---
+format: 
+  revealjs: default
+engine: knitr
+_quarto:
+  tests:
+    revealjs:
+      ensureHtmlElements:
+        - ["#verbatim div.sourceCode pre code", "#embed div.sourceCode pre code"]
+        - ["#verbatim div.cell", "#embed div.cell"]
+      ensureFileRegexMatches:
+        - ["this is code\\.R"]
+        - []
+---
+
+From issue : https://github.com/quarto-dev/quarto-cli/issues/4712
+
+## Verbatim {#verbatim}
+
+````{verbatim}
+Some content
+````
+
+## Embed {#embed}
+
+````{embed, file = "code.R"}
+````
+