Migrate demo from CRA to Vite

[axelboc]

Tested on FF 60 (i.e. the "browser not supported" error shows up as expected).

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EDIT - Summary of changes:

• The demo is now based on Vite instead of CRA and no longer requires a config-override.js file to work.
• Environment variables REACT_APP_DIST and STORYBOOK_DIST have been removed, as testing distributed packages this way is not sufficiently reliable and not worth the complexity it adds to the monorepo. Distributed packages can be tested by publishing beta versions. The four Codesandboxes (Add CRA and Vite sandboxes and fix @h5web/lib stylesheet export #938) are available to test the published packages reliably inside both CRA and Vite apps before publishing official releases.
• The process of building the packages' styles is now more robust and a lot clearer. It is performed after the JS build and is based on a separate Vite config (vite.styles.config.js) and a separate entrypoint (src/styles.ts).
• Consumers of @h5web/app now need to import a single stylesheet instead of two.
• Building the packages, their styles and their type declarations, can now be done by running a single script: pnpm packages (no need to run pnpm packages:dts afterwards, as this is now done automatically).

[axelboc]

Alright, so as mentioned in the chat, removing the global styles imports from the index.ts files of the packages means that the packages' output stylesheets dist/style.css no longer include said global styles...

Instead of trying to find a sub-optimal solution that works for both the dev/build of the demo/Storybook and the build of the packages, I decided to take care of the problem on its own.

In the process, I managed to solve the problem of having to import two stylesheets when consuming the @h5web/app package. Now consumers just have to import @h5web/app/dist/styles.css (note the s). Also worth noting that I now run the build:dts script from the build script, so I've removed the packages:dts script from the root package.json.

I've explained how the packages are now build in details in the CONTRIBUTING guide. Hopefully it is clear. Relevant files (inside each package) are: package.json, vite.styles.config.js and src/styles.ts, README.md. Relevant commit: 71ae427

[loichuder]

👏