[x] do not embed font into SVG when embedding Excalidraw into other Excalidraw
[x] add ```html <SVG>...</SVG> ``` codeblock to excalidraw markdown
[x] read pre-saved `<SVG>` when generating image preview
[x] update code to adopt change files moving from AppState to App 
- Add "files" to legacy excalidraw export

[x] PNG preview
[x] markdown embed SVG 190
[x] markdown embed PNG
[x] embed Excalidraw into other Excalidraw