Add support for theme family-specific syntax mapping overrides (#3551)

This PR adds support for adding a specific set of mappings from Zed
syntax tokens to VS Code scopes for a particular theme family.

We can use this as a fallback when we aren't otherwise able to rely on
the mappings in the theme importer, as sometimes it isn't possible to
make a specific enough matcher that works across all of the themes.

Release Notes:

- N/A

assets/themes/src/vscode/rose-pine/family.json

@@ -17,5 +17,8 @@
       "file_name": "rose-pine-dawn.json",
       "appearance": "light"
     }
-  ]
+  ],
+  "syntax": {
+    "function": ["entity.name"]
+  }
 }