Fix broken JupyterWidget chart by pinning Vega (#3269)

* Use latest Vega from esm.sh and pin Vega-Lite to the version Altair is expecting.

* Add note to NOTES_FOR_MAINTAINERS.md to update the Vega-Lite version in JupyterChart.

NOTES_FOR_MAINTAINERS.md

@@ -49,6 +49,15 @@ of some docstrings.
 Major version updates (e.g. Vega-Lite 1.X->2.X) have required substantial
 rewrites, because the internal structure of the schema changed appreciably.
 
+## Updating the Vega-Lite in JupyterChart
+To update the Vega-Lite version used in JupyterChart, update the version in the esm.sh URL in `altair/jupyter/js/index.js`.
+
+For example, to update to Vega-Lite 5.15.1, Vega 5 and Vega-Embed 6, the URL should be:
+
+```javascript
+import embed from "https://esm.sh/vega-embed@6?deps=vega@5&[email protected]";
+```
+
 ### Updating vl-convert version bound
 When updating the version of Vega-Lite, it's important to ensure that 
 [vl-convert](https://github.com/vega/vl-convert) includes support for the new Vega-Lite version. 

altair/jupyter/js/index.js

@@ -1,5 +1,5 @@
-import embed from "https://cdn.jsdelivr.net/npm/vega-embed@6/+esm";
-import debounce from "https://cdn.jsdelivr.net/npm/[email protected]/debounce/+esm";
+import embed from "https://esm.sh/vega-embed@6?deps=vega@5&[email protected]";
+import debounce from "https://esm.sh/[email protected]/debounce";
 
 export async function render({ model, el }) {
     let finalize;