[TASK] Define readonly for never-written private TypeScript variables

IDEs like PhpSstorm complain about the same thing,
so better we have that defined and checked via CI.

Commands executed:
  node_modules/.bin/eslint --fix Sources/TypeScript/

Resolves: #101780
Releases: main, 12.4
Change-Id: I01e0289b671dfd4348319bca90dddaec085fbfe4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80736


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Benjamin Franzke <ben@bnf.dev>