Switch to composer mode in running instance

Migrating TYPO3 from Classic Mode to Composer Mode requires at least the following steps:

1. Write down the current version of TYPO3 and all extensions
2. Remove all embedded TYPO3 and extension code incl. Git submodules
3. Add a Composer manifest
4. Add the Composer vendor-dir (and bin-dir if custom) to your .gitignore

5. Require TYPO3 and all extensions with the versions and --prefer-lowest, e.g.

   composer require typo3/cms:^8.7.7 --prefer-lowest
   

   This ensures that you don't accidentally perform updates before completing the switch.

Since no further changes to user files or database data is required you will be running TYPO3 in Composer Mode now.

Afterward you will most likely also need to adapt your deployment workflow to ensure at least one composer install is executed after deploying a new version.