MAINT: upgrade to webpack5

[agoose77]

@mmcky this PR should update the JS dependencies to webpack5. I haven't visually tested this, only that it compiles, so it would be good if you could give this a once over on an existing site that uses the theme.

The main changes were:

• Upgrading dependencies to webpack 5
• Dropping node-sass
• Replacing file-loader with mini-css-extract-plugin
• Replacing @import with @use (https://sass-lang.com/documentation/at-rules/import/)
• Drop minification config (automatic now)

Most of these are in sphinx-book-theme, but I see from this PR that I now have some additional work to do!

There's a chance I've made a mistake here, given that I haven't compared the outputs to an existing deployment; if you'd be able to do that, would be much obliged.

[mmcky]

thanks @agoose77 i will dive in and take a look.

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[mmcky]

@agoose77 I fixed up the pre-commit but not sure why they haven't flowed through to this PR.

[mmcky]

thanks @agoose77 -- really appreciate the help here.