Fix embed and verbatim knitr's engine for revealjs

[cderv]

General fix : With new embed feature in 1.3 for Jupyter, we need to make sure this does not override embed Knitr engine (cc @dragonstyle as discussed)

Specific to revealjs: Set echo = TRUE for embed and verbatim engine when revealjs because those engine expect echo to be TRUE to be shown, but revealjs default to FALSE

This fix #4712

Note: our extension to include external file works fine - but it feels like those engines should still work correctly

By working on these R files, I think we can improve the handling of chunk hook, especially to treat those two engines as others (if we want to support echo: fenced for example. At first I did it in this PR, but then as this is some R refactoring more generic, I'll do it in another PR (#4735). We can then postpone to 1.4 if we think this is two much.

[cderv]

Following meeting and discussion with @cscheid this needs to be split as the embed override is in fact a more broader issue where non-cell handler should not be passed to knitr among handledLanguages

Currently all are

names(knitr::knit_engines$get())

 [1] "awk"       "bash"      "coffee"    "gawk"      "groovy"    "haskell"  
 [7] "lein"      "mysql"     "node"      "octave"    "perl"      "php"      
[13] "psql"      "Rscript"   "ruby"      "sas"       "scala"     "sed"      
[19] "sh"        "stata"     "zsh"       "asis"      "asy"       "block"    
[25] "block2"    "bslib"     "c"         "cat"       "cc"        "comment"  
[31] "css"       "ditaa"     "dot"       "embed"     "eviews"    "exec"     
[37] "fortran"   "fortran95" "go"        "highlight" "js"        "julia"    
[43] "python"    "R"         "Rcpp"      "sass"      "scss"      "sql"      
[49] "stan"      "targets"   "tikz"      "verbatim"  "ojs"       "mermaid"  
[55] "include"  

Meaning include is also pass (see the end) - embed is there as an override of the built-in one, like dot handler from Quarto which overrides the knitr one, but I think that is fine in the context of Quarto.

So we need first another PR for the handler adaptation

[cscheid]

lgtm!