Make assets:install smarter with symlinks

| Q             | A
| ------------- | ---
| Bug fix?   | no
| New feature?      | yes
| BC breaks?      | no
| Deprecations?      | no
| Tests pass?      | -
| Fixed tickets     | #11297
| License     | MIT
| Doc PR | -
This looks perfect, but I *don't* think that it'll catch the case where you're on a VM with a Windows host machine, you're sharing the Symfony directory with the host machine, and you don't have proper permissions to actually create symlinks on the Windows machine (see: https://github.com/symfony/symfony/issues/11297#issuecomment-48075759).

The reason I don't think that this will work for that one case is that I don't think these users are currently seeing an `IOException` - the `assets:install` just *thinks* that symlinks work (I could be wrong here, but pretty sure I'm not).

So, I think we need to actually *check* for the integrity of the symlink - e.g. can you look inside of its contents or not. This *could* actually go into the `Filesystem::symlink` method, if we could find a clean way, consistent way of doing this check.