chore: remove legacy internal eslint configs

[mainframev]

• Removed legacy internal eslint configs
• Removed a few leftovers with FlatCompat old legacy FUI configs
• Updated README for external eslint-plugin to mention flat configuration example

[github-actions]

📊 Bundle size report

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[github-actions]

Pull request demo site: URL

[mainframev]

discussed in team channel, this is needed for bypassing issue with yarn.lock:

Problem

When modifying dependencies in this repo, yarn install sometimes consolidates npm alias entries in yarn.lock into a format that breaks --frozen-lockfile validation on CI:

Broken format (consolidated):

"string-width-cjs@npm:string-width@^4.2.0", string-width@^4.1.0, string-width@^4.2.0, string-width@^4.2.3:

Working format (separate):

"string-width-cjs@npm:string-width@^4.2.0":
...
string-width@^4.1.0, string-width@^4.2.0, string-width@^4.2.3:
...

Root Cause
The @isaacs/cliui package (used by jackspeak → glob → jest, mocha, etc.) declares npm aliases (string-width-cjs, strip-ansi-cjs, wrap-ansi-cjs). Yarn 1.x has a bug where --frozen-lockfile fails to validate these consolidated alias entries, even though yarn install without the flag works fine.

Previous Fix

This was already fixed in PR #35611 (commit a33772c), but running yarn install can reconsolidate the entries.

[dmytrokirpa]

could we use resolutions for the string-width-cjs/string-width packages affected instead of their dependent?

[mainframev]

Hm, don't think it will work as string-width-cjs is not real package name and just alias. By locking jackspeak we are avoiding @issacs/cliui which brings those aliased packages