[TASK] Re-add extension-key to styleguide's composer.json

The extension key setting was accidentally dropped in one
of the monorepo merge preparation changes:
https://github.com/TYPO3-CMS/styleguide/pull/475

Also reformat to use tabs as in other composer.json files.

Resolves: #102309
Releases: main
Change-Id: I97f6864551b524b27e770a5a046a77744fedf5e3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81674


Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Jasmina Ließmann <minapokhalo+typo3@gmail.com>