feat: add output language support for code blocks

[natrimmer]

Add --output-lang flag to exec command

When the output of a command is source code (e.g. cat main.go), the default output fence language doesn't give you syntax highlighting. This adds an optional --output-lang flag to exec that sets the fence language on the output block:

showboat exec demo.md bash --output-lang go "cat main.go"

produces:

```go
package main
...
```

instead of:

```output
package main
...
```

The default behavior is unchanged -- without the flag, output blocks still use output. The parser handles both formats, and extract round-trips the flag correctly.