Allow URLs and multiple custom panel styles/scripts

[lukaskleinschmidt]

Description

While working on a solution for this issue of my vite plugin I came across some shortcomings of the current custom panel styles/scripts implementation.

The shortcomings:

1. You can only have a path to a file
2. You can only define one file respectively

What would be nice to have:

1. URL support for custom panel assets
2. The possibility to add multiple assets

Reasons
In this case vite specific but most likely not limited to that.
To be able to make use of vites dev server or hot reloading you need to be able to define a full URL to work when developing. In addition to that you need to include the @vite/client to make hot reloading work. This requires you to include multiple script assets when using a custom panel.js as well.

Right now I use the panel.route:after hook to inject the tags with a regex to make it work. But in general I think this might be useful outside of vite as well.