markdown: Handle definition lists in parser (#17617)

Resolves https://github.com/zed-industries/zed/issues/17607.

This PR makes it so the Markdown parser can handle Markdown containing
definition lists.

Note that this is just parser support, we aren't yet doing anything with
the definition lists themselves.

Release Notes:

- N/A

crates/markdown/src/parser.rs

@@ -232,6 +232,10 @@ pub enum MarkdownTag {
 
     /// A metadata block.
     MetadataBlock(MetadataBlockKind),
+
+    DefinitionList,
+    DefinitionListTitle,
+    DefinitionListDefinition,
 }
 
 #[derive(Clone, Debug, PartialEq)]
@@ -317,11 +321,9 @@ impl From<pulldown_cmark::Tag<'_>> for MarkdownTag {
             },
             pulldown_cmark::Tag::HtmlBlock => MarkdownTag::HtmlBlock,
             pulldown_cmark::Tag::MetadataBlock(kind) => MarkdownTag::MetadataBlock(kind),
-            pulldown_cmark::Tag::DefinitionList
-            | pulldown_cmark::Tag::DefinitionListTitle
-            | pulldown_cmark::Tag::DefinitionListDefinition => {
-                unimplemented!("definition lists are not yet supported")
-            }
+            pulldown_cmark::Tag::DefinitionList => MarkdownTag::DefinitionList,
+            pulldown_cmark::Tag::DefinitionListTitle => MarkdownTag::DefinitionListTitle,
+            pulldown_cmark::Tag::DefinitionListDefinition => MarkdownTag::DefinitionListDefinition,
         }
     }
 }