Desktop: Fixes #6719. Avoid reloading loaded plugins

[SeptemberHX]

This PR fixes #6719. It ignores the loaded plugins to avoid duplicate css.

Tested with the following plugins:

Before:

After:

[laurent22]

Looks good, thank you!

[laurent22]

@SeptemberHX, I think this change is now preventing content scripts from loading, when they should actually load. See this bug report there: https://discourse.joplinapp.org/t/pre-release-v2-9-is-now-available-updated-23-october/26416/57?u=laurent

The fact that the state is set outside the hook is a code smell anyway, and probably the source of this bug.

Do you have any suggestion on how to fix this?

[SeptemberHX]

I reproduced this bug, and it works again after reverting this commit. Sorry for this problem.

After moving const loadedPluginIdSet = new Set<string>(); inside useExternalPlugins(), it still doesn't work. All the plugins needed to be reloaded after switching between notebooks and return from setting page, and the loadedPluginIdSet is also reset when initializing useExternalPlugins.

However, it seems all the js and others can be loaded without problem even when they are loaded, but css is not really necessary needed to be reloaded. And it works for me to remove

if (loadedPluginIdSet.has(contentScript.id)) {
	continue;
}

and move it to:

if (!loadedPluginIdSet.has(contentScript.id)) {
	if (mod.assets) {
		const cssStrings = [];
                 //...
	}
}