Add eslint rule to stop using index file for exporting

[michenly]

Motivation

Having index file while convenient, leads to performance issue in test and build, especially in a ESM world.
We should stop using component index all together and write a eslint rule that stop it.

There is already a decision for Shopify/web top stop using component/index but it really should be stop everywhere
https://github.com/Shopify/web/blob/master/documentation/decisions/09%20-%20Explicit%20root%20components%20imports.md

Propose Rule

While the actual reason to stop using index file have to do with the number of files being check.
(example:
a file that imports 1 other file that imports 1 other file that imports 1 other file that imports 1 other file that imports 1 other file that imports 1 other file that imports 1 other file that imports 1 other file = 6 total files
vs 1 file that imports 4 files that don't import anything from there = 5 total files

both will be equally bad on performance)

The biggest offender to those are currently component/index or any index file that export a lot of things, because the chances are not all the object being exported will be use upon import.

So I propose that we add a rule to stop any index file that export more than 10 things, and make the this arbitrary number editable in the lint rule.

https://github.com/Shopify/web-configs/blob/main/packages/eslint-plugin/docs/rules/strict-component-boundaries.md and
https://github.com/Shopify/web-configs/blob/main/packages/eslint-plugin/lib/rules/images-no-direct-imports.js
both of those rules are in direct conflict with what is being propose and should be update/delete/adjusted at the same time.

no-restricted-imports used in web and https://github.com/Shopify/web-configs/blob/dab62e4f72e14c1f33ae6a6a626f890e2a872f3e/packages/eslint-plugin/docs/rules/no-ancestor-directory-import.md could also be rules that we look into