feat(cheddar): Set front_matter_delimiter = '---' extension

This chops off "front matter", which we have - for example - in Tvix
documentation that is rendered through Pandoc.

Change-Id: I65c7a4ad8171423a50aaf47c046dab8c774346d8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2717
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tools/cheddar/src/main.rs

@@ -49,6 +49,7 @@ lazy_static! {
         options.extension.header_ids = Some(String::new()); // yyeeesss!
         options.extension.footnotes = true;
         options.extension.description_lists = true;
+        options.extension.front_matter_delimiter = Some("---".to_owned());
 
         // Required for tagfilter
         options.render.unsafe_ = true;