[Feature Request] "Show hidden lines" functionality for non-rust files

[rococode]

As far as I can tell, there is no way to have the "show hidden lines" thing when showing a subset of lines without using #rustdoc_include.

However, #rustdoc_include wraps invalid rust (i.e. any file that's actually not supposed to be rust code) automatically with a main function.

For example, something like,

```rust
{{#rustdoc_include hello.txt:1:2}}
\```

will render as:

#![allow(unused)]
fn main() {
hello
}

when "Show hidden lines" is clicked, which interferes with copy-pasting and general cleanliness. It would be great if the "Show hidden lines" / "Hide lines" functionality could be toggled on somehow for regular non-rust files for which only a certain range of lines are shown by default.

#1294 and #1289 seem related, though they appear to still be within the context of rust code files.