Enable build system to reload the web-app during dev

[bric3]

Currently the web-app development is a bit awkward the web-app can be run and developed in a standalone way, and to run this web-app in the plugin, we currently need to run the build. At this time the web-app is packaged, and copied on the excalidraw plugin, then the plugin is run. This prevents the reload/refresh of the web-app.

Ideally during development the plugin could look for an alternative path to load the sources, this would allow to work on the web-app without rebuilding the plugin.