WIP: Only load embed_chart JS for markdown if it's required

What does this MR do?

Use async import for embed_group JS, so it only gets loaded if we actually have charts to render. Approach is the same as render_mermaid.js

Hopefully, this means monitoring + echarts is no longer included in main.js (I have not confirmed this as keep getting OOM errors when trying to run bundle analyzer :( )

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides