Add barrel export

[cefn]

This modification (to add barrel exports) emphasises that there was still a failing in the bundling and resolution strategy. The previous solution only got the NextJS bundling process as far as the .ts files, and the stupidity of the convention to require .js files was defeating the bundler's attempts to resolve source.

See microsoft/TypeScript#49083 (comment) for the original intransigent position taken by the Typescript team and then see microsoft/TypeScript#59767 for when they (very recently) saw the light!

This means we can add .ts file paths in imports, assuming we add the following to our tsconfig.json compilerOptions

    "allowImportingTsExtensions": true,
    "rewriteRelativeImportExtensions": true