[internal] Generate index.ts to keep it up to date

Summary 💡

It's easy for the global index.ts to get out of date with new components added to the repo - I just created https://github.com/mui/material-ui/pull/33982 to fix the current mismatch in @mui/joy. It's very easy to generate the index based on some rules in the directory naming - this way (with a "golden" test - checking that the submitted code matches the generated one) the index should never be out of date.

Current index.ts can be generated by adding:

export { default as colors } from './colors';
export * from './styles';

followed by one of these for each directory staring with [A-Z]:

export { default as DirName } from './DirName';
export * from './DirName';

Random notes:

• this probably could be done for other packages as well, I'm guessing @mui/joy is under most active development, that's why it got out of sync
• it's probably a good idea to add a comment at the top of the file saying something "this file is generated, run <command> to update it"

I'm happy to make a PR with this script, but would love to hear your thoughts first.

Examples 🌈

No response

Motivation 🔦

No response

Start with packages/mui-joy/src/index.ts and inspect how its exports correspond to the package directories whose names begin with uppercase letters. Define the generation rules described in the issue, then verify that a golden test can detect when the checked-in index differs from generated output; done means the index stays synchronized and documents how to regenerate it.