preserve trailing \n in source code by doubling it, because later we …

…will remove trailing \n in fenced_block()

one example is here: https://github.com/PRQL/prqlc-r/blob/d3606f09b4efda020967c2b8518ea90a830a63e0/R/knitr-engine.R#L51 sql_code contains a trailing \n, but prql_code doesn't, so

```
sql_code
```

will generate

```
prql_code```

after substitution, which is wrong Markdown output

R/hooks-md.R

@@ -186,7 +186,7 @@ hooks_markdown = function(strict = FALSE, fence_char = '`') {
   list(
     source = function(x, options) {
       x = hilight_source(x, 'markdown', options)
-      if (strict) hook.t(x) else hook.r(x, options)
+      if (strict) hook.t(x) else hook.r(sub('\n$', '\n\n', x), options)
     },
     inline = function(x) {
       if (is_latex_output()) .inline.hook.tex(x) else {