[TASK] Cleanup .gitignore

Make sure root level excludes are prefix with
a forward slash (/).

Sort the .gitignore into logical groups.

Releases: 7.6, 8.7, master
Resolves: #81992
Change-Id: If1cb4105e57fb7ab3c28448abc3eaeb898da60e3
Reviewed-on: https://review.typo3.org/54113


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>