Add support for ```java snippets in Markdown inputs

[Gedochao]

Closes #1602
Closes #1601

Rather than the scala and scala raw snippets we have for Scala in Markdown (which mirror .sc and .scala inputs, respectively), I believe we'll just have java snippets for .java inputs. This is due to wrapping script-like Java snippets being difficult, due to how imports and other syntax is handled. We could simply wrap code in a main method, but that seems too limiting in comparison to what we have in Scala.

Perhaps we could have jshell snippets and/or .jsh input support at some point, as that would deliver some of what we could possibly want from Java scripts... Eespecially as we now have JShell support in the REPL. But that is definitely out of scope here (and I'm not sure how feasible, either).

And so, this change allows for markdown inputs like this:

## Some Markdown Stuff
blah blah Java snippet
```java
public class Snippet {
  public static void main(String[] args) {
    System.out.println("Hello");
  }
}
```

Checklist

• tested the solution locally and it works
• ran the code formatter (scala-cli fmt .)
• ran scalafix (./mill -i __.fix)
• ran reference docs auto-generation (./mill -i 'generate-reference-doc[]'.run)

How much have your relied on LLM-based tools in this contribution?

Extensively, Cursor + Claude

How was the solution tested?

new automated tests