vendor directory based typo3 installation

Hi!

I propose to enable a typo3 installation in 'composer mode'; meaning everything automatic goes into the vendor directory. Like the linked branch, that switches to 'vendor directory mode' if the 'extra' 'typo3_vendor_based' is set.

Why?

Because eveything outside of 'vendor' is supposed to be editable, i.e. NOT composer influenced. I don't like composer messing in MY sources ;)

That's also why I changed the installation target for composer-installed extensions (typo3/ext). I know this 'mixes' two different vendor directories, but isn't creating a third destination (a common ext directory used by core and extensions) worse?

In my preliminary tests, I could easily set up a typo3 site with 'only' a link to typo3 core messing in my sources and all extensions linked as system extensions.

Of course, I'm open to improvements/suggestions.

KampfCaspar