Add missing d2 and graphviz entries to extensions.toml (#1540)

This PR adds the missing entries for the `d2` and `graphviz` extensions,
since they were missed in these PRs:

- #1534
- #1536

extensions.toml

@@ -282,6 +282,10 @@ version = "0.2.0"
 submodule = "extensions/d"
 version = "0.0.8"
 
+[d2]
+submodule = "extensions/d2"
+version = "0.1.0"
+
 [dafny]
 submodule = "extensions/dafny"
 version = "0.0.1"
@@ -513,6 +517,10 @@ version = "0.2.0"
 submodule = "extensions/graphql"
 version = "1.0.0"
 
+[graphviz]
+submodule = "extensions/graphviz"
+version = "0.1.0"
+
 [green-monochrome-monitor-crt-phosphor]
 submodule = "extensions/green-monochrome-monitor-crt-phosphor"
 version = "0.1.3"