Panic with setext heading containing text and JSX tag without matching close

[begleynk]

Found a panic with this very specific input:

Hi <>
=====

Resulting panic:

thread 'main' panicked at src/to_mdast.rs:1249:9:
internal error: entered unreachable code: expected heading on stack
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Requires parsing with MDX options:

    println!(
        "{:?}",
        markdown::to_mdast(
            "Hi <>\n======",
            &markdown::ParseOptions::mdx()
        )?
    );

I checked back to alpha 16 and the crash is still there, so doesn't look like this is a recent regression.

I can try to take a look into what's going on here, but likely can't get to it until next week earliest.