feat: add -D CLI option to enable definition lists

[ytmimi]

No description provided.

[Martin1887]

This change has created a compilation error in dos-fuzzer, please check the build errors and fix the problem to get this merged. Thanks!

[ytmimi]

Happy to make the change, though I think nightly CI is failing because of rust-lang/rust#131833 and not this PR. This is the error from the logs:

error: unused return value of `exec` that must be used
   --> dos-fuzzer/src/main.rs:282:25
    |
282 |                         Command::new(&args[0]).args(&args[1..]).exec();
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D unused-must-use` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(unused_must_use)]`
help: use `let _ = ...` to ignore the resulting value
    |
282 |                         let _ = Command::new(&args[0]).args(&args[1..]).exec();
    |                         +++++++

error: could not compile `dos-fuzzer` (bin "dos-fuzzer") due to 1 previous error

[ollpu]

This clashes with -D i.e. --enable-subscript

[ytmimi]

@ollpu I don't think --enable-subscript was available when I put this PR together.

[ytmimi]

@ollpu yeah, looks like #966 configured -D for --enable-subscript, when it was already being used for definition lists.

[ollpu]

Ah, my bad, I just went by the order they were in --help. We'll get this sorted