fix: explicitly set publicPath for static assets

[sebjacobs]

https://github.com/RaspberryPiFoundation/experience-cs/issues/23

We recently embedded the scratch-gui within our Experience CS app.

We have since noticed a number of issues with assets failing to load within the browser, for example the browser is attempting to load assets from URLs relative to the webpack entry point (rather than from the root or from /scratch-gui/).

After some investigation it turns out that is due to the fact that the webpack option publicPath had not been explicity set [1] (which apparently default to auto).

From digging into the webpack docs [2], it looks like when publicPath is set to auto it will attempt to load assets relative to the entrypoint which is not what we want.

For our purposes we want assets to be served from /scratch-gui, and I can't see the need to make this configurable at this stage, therefore I have hardcoded this value for now as that also makes building the package less prone to error.
However if we were to push changes upstream, then I think it would make sense to use an ASSET_PATH environment variable.

Rather than configuring this globally, I have opted for defining it within the Asset module definition (nested under the generator option) as that makes it clearer that this option is only really relevant for those assets thus making it easier for future developers to understand and make changes in the future, especially for those who are less familiar with the intricacies of webpack config.

I also dug into the latest version of scratch-gui and scratch-webpack-configuration and can confirm that this would still be an issue even with the latest releases, but at least they are now explicit about setting publicPath to auto, rather than the config option missing entirely and falling back to the default value of auto [3].

[1]

https://github.com/scratchfoundation/scratch-webpack-configuration/blob/v1.5.1/src/index.cjs#L71
https://github.com/RaspberryPiFoundation/scratch-gui/blob/b723be4d1/webpack.config.js#L20

[2]

https://webpack.js.org/guides/public-path/

**Automatic publicPath**

There are chances that you don't know what the publicPath will be in advance,
and webpack can handle it automatically for you by determining the public path
from variables like import.meta.url, document.currentScript, script.src or self.location.

[3]

https://github.com/scratchfoundation/scratch-gui/blob/853caca41/webpack.config.js#L29

[floehopper]

Great detective work - I found the scratch-webpack-configuration code/config very confusing when I looked at it previously!

This looks good to me - assuming the build errors can be resolved 👍

I think we should probably have a wider discussion about how much effort we're going to put in to making our changes upstream-able. e.g. I notice that the recent change to set the default stage size is hard-coded rather than configurable.