Add terminating \n to comment src

Normally, the src attributes of evaluate output has trailing `\n`:
```
> evaluate::evaluate(input = "ls()\nls()")
[[1]]
$src
[1] "ls()\n"

attr(,"class")
[1] "source"

[[2]]
[1] "character(0)\n"

[[3]]
$src
[1] "ls()"

attr(,"class")
[1] "source"

[[4]]
[1] "character(0)\n"
```

But not for comments:
```
> evaluate::evaluate(input = "#foo\n#bar")
[[1]]
$src
[1] "#foo"

attr(,"class")
[1] "source"

[[2]]
$src
[1] "#bar"

attr(,"class")
[1] "source"
```

This is causing staticdocs to render "dontrun" examples without hard returns at all (I can show you an example if you want).

With this change, `\n` is appended to each comment line. Note that the final line (`#bar` in the example above) will have a trailing `\n`, unlike the first example (`ls()\nls()`).