Keep non-mdoc code block directives

[Jisoo Park]

Suppose that I want to add Docusaurus title to an mdoc code block like this:

```scala mdoc title="Main.scala"
object Main extends App {
 // ...
}
```

Expected output

```scala title="Main.scala"
object Main extends App {
 // ...
}
```

Actual output

```scala
object Main extends App {
 // ...
}
```

Alternative case

In this case mdoc directive is not recognized.

```scala title="Main.scala" mdoc
object Main extends Ap { // wrong but mdoc doesn't report error
 // ...
}
```

[Tomasz Godzik]

Thanks for reporting! I can't work on it right now, but if anyone is willing I can try and help out.

[Jisoo Park]

@tgodzik I want to tackle this in my spare time. Could you give me some quick advice on where to look at?

[Tomasz Godzik]

The tag data should be kept in CodeFence

and it seems it's crated in that file. What I would do personally is add a test to:
https://github.com/scalameta/mdoc/blob/main/tests/unit/src/test/scala/tests/markdown/MultiModsSuite.scala

and print/debug what is going with that CodeFence and how are the additional modifiers being handled.

Tests can be run best with:
sbt and then
~unit/testOnly tests.markdown.MultiModsSuite to run the whole suite or ~unit/testOnly tests.markdown.MultiModsSuite -- *my-new-test-case to run the added case. ~ will rerun the test on changes so it's super useful for quick debugging.