get-chunk-name doesn't work in Webpack v5.

[davidwarrington]

Code splitting has been ported from Slate v1 in feature/code-splitting, but has required downgrading Webpack to v4.

The getChunkName function sometimes returns names that are the same as existing build entrypoints. This behaviour has been deprecated in Webpack 5, so an alternative solution will probably be required in the future.

Some possible ways forward are:

• Swap to l. and t. prefixes - This results in extra modules being created (for example any module that would previously be merged with layout.theme is now a separate l.theme), but should be safe otherwise.
• Use default names - This results in outputs like 143.js. Like above, this would also result in extra modules being created, but could also resolve the "ENAMETOOLONG" issue. This will only be possible if HTMLWebpackPlugin allows checking each module for entrypoint names.

[davidwarrington]

Package has now been set up to use Webpack v5 codesplitting. Might need to revisit this further down the line.