migrate to ESLint flat config, convert to ESM

[Simek]

Why

Refactor and migrate to flat config ESLint v9 setup. Convert monorepo to ESM to leverage properly minimal required Node version and simplify linting.

How

Upgrade ESLint to v9, convert setup to flat config, remove redundant stand-alone Prettier checks.

In the process, I have also:

• reorganized the repository structure, so all scripts are placed in scripts directory, and all monorepo packages resides in packages directory
• fixed lint-examples packages scripts, and formatted ./docs files using it
• fixed sync-api-docs packages, it runs correctly, but looks like no files in core repo contain @generate-docs annotation anymore
• fixed and re-enabled remark-lint-no-broken-external-links check plugin for unversioned docs via new script which should be run locally
• fixed and hooked up image-check script
• fixed Snack remark plugin tests

Future TODOs

• fix LLMS generation (it's not working properly on PROD, see https://reactnative.dev/llms.txt)
• figure out SCSS linting (@eslint/css cannot parse SCSS files yet), for now we only use stand-alone Prettier to format the files

Test plan

All lint checks and tests across monorepo are working and do not yield any errors. Website apps runs and builds as expected.

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[clicktodev]

can you make sure ide detection still works? because the eslint extension in vscode sometimes stops working when migrating to eslint v9

here's an example

[Simek]

Thanks for bringing this up! 👍 Usually, in-IDEs detection problems reflects the issues in the lint config definition, or monorepo/workspace setup on users machine.

For this PR, I have tested the following editors (also with MD/MDX content, not only for JS/CJS/MJS/TS as seen on screenshots below):

• JetBrains IDEA

• VS Code

• Zed

[clicktodev]

awesome, it seems as long as the eslint config is exported from the root of the project then it's correctly detected by IDEs, so in the case of nextra i needed to reexport from root it because it's defined in a package

[robhogan]

This is some really nice cleanup and modernisation, thanks!