[BUGFIX] Generate correct asset symlinks on Windows

This patch fixes two issues when generating symlinks to the public
extension resources (Public/Resources) into the _assets folder during
Composer installation.

First, directory symlinks can't be created the same way on Windows as
on Unix platforms. The creation requires elevated rights, and the
required command is named differently as well. Junctions should be used
on Windows instead, because they do not need elevated privileges to be
created.

When installing a TYPO3 extension via Composer, its Resources/Public
folder is getting symlinked to the publicly accessible _assets folder.
Until now, this process only used the Unix specific functions provided
by Composer. It did not create any symlinks on Windows based
installations, without generating any error message.

This patch adds the same checks and platform-specific functions used by
Composer when symlinking packages from a local path repository.

The second issue that has been addressed is the naming ...