Add some useful Rmd snippets

[jooyoungseo]

What problem did you solve?

Closing #1008

(If you have)Screenshot

(If you do not have screenshot) How can I check this pull request?

1. Open Rmd file.

2. Insert the following snippets:

• inline r code

• inline footnotes

• speaker notes

• two columns

• reference list

• code chunk

• incremental list

• nonincremental list

• incremental fenced block

• nonincremental fenced block

• unordered item

• ordered item

• task list

• checked item

• unchecked item

• strikeout

• superscript

• subscript

• bookdown theorem

• cross-reference bookdown theorem

• bookdown lemma

• cross-reference bookdown lemma

• bookdown corollary

• cross-reference bookdown corollary

• bookdown proposition

• cross-reference bookdown proposition

• bookdown conjecture

• cross-reference bookdown conjecture

• bookdown definition

• cross-reference bookdown definition

• bookdown example

• cross-reference bookdown example

• bookdown exercise

• cross-reference bookdown exercise

• bookdown hypothesis

• cross-reference bookdown hypothesis

• bookdown equation

• cross-reference bookdown equation

• figure chunk

• external figure chunk

• cross-reference bookdown figure

• cross-reference bookdown table

• bookdown section label

• cross-reference bookdown section

• bookdown part

• bookdown unnumbered part

• bookdown appendix

[jooyoungseo]

Also added all the supported languages to Insert code chunk snippet. Users now can interactively choose the supported languages.

The following languages are currently available:

names(knitr::knit_engines$get())
#>  [1] "awk"       "bash"      "coffee"    "gawk"      "groovy"    "haskell"  
#>  [7] "lein"      "mysql"     "node"      "octave"    "perl"      "psql"     
#> [13] "Rscript"   "ruby"      "sas"       "scala"     "sed"       "sh"       
#> [19] "stata"     "zsh"       "asis"      "asy"       "block"     "block2"   
#> [25] "bslib"     "c"         "cat"       "cc"        "comment"   "css"      
#> [31] "dot"       "embed"     "fortran"   "fortran95" "go"        "highlight"
#> [37] "js"        "julia"     "python"    "R"         "Rcpp"      "sass"     
#> [43] "scss"      "sql"       "stan"      "targets"   "tikz"      "verbatim" 
#> [49] "glue"      "glue_sql"  "gluesql"

^{Created on 2022-02-21 by the reprex package (v2.0.1)}

stan and sql snippets have followed the recommended templates by R Markdown: The Definitive Guide.

[renkun-ken]

We are about to release a new version. Would you like to include this PR in the upcoming release?

[jooyoungseo]

I will have more time tomorrow to finish this PR if that works for you. I don't mind if you merge this after the new release for the next round.

[renkun-ken]

LGTM Thanks!