feat: New modular config for (and with) ESLint v9 support

[susnux]

• Resolves eslint 9 #831
• Closes [WIP] chore(config): migrate to new eslint config #441
• Also included updated stylistic rules to follow our code styles, ref Frontend code style  nextcloud/standards#11

Summary

This provides a modular build configuration in flat config format (ESLint v9) as v8 is now EOL.
Please note that this changes some formatting and some warnings might be different, as standard-js seems to be dead and instead we use now more popular plugins / do it our self according to our code styles.

Mostly you might see more formatting or arrays to git diff friendly multi-line versions.

TODO

Do we want to move the nextcloud plugin simply to this repository? Both have more or less a one-to-one relation ship, so maintaining both packages does not make that much sense for me.
So either:

1. Merge the nextcloud eslint plugin into this repository in the plugins directory
2. Keep external, then we need to update it before merging this.

[skjnldsv]

Jesus, what a beast of a PR 😅

[skjnldsv]

Do we want to move the nextcloud plugin simply to this repository? Both have more or less a one-to-one relation ship, so maintaining both packages does not make that much sense for me.

For this specific use case, might be worth it yep