[Node] Stop deleting NODEFS mount directories on unmount

[adamziel]

Motivation for the change, related issues

Fixes a Playground CLI crash that happened when rotating a PHP instance while using a local mount for /wordpress.

Implementation details

Imagine you're mounting a local directory, e.g. /home/adam/my-site at /wordpress in Playground's VFS. By default, /wordpress does not exist so createNodeFsMountHandler() will create it. When it's time to unmount, createNodeFsMountHandler() remembers it created /wordpress just for mounting and will attempt to clean it up. However, since /wordpress is also a CWD, the filesystem will refuse to delete it by throwing error 10 EBUSY.

This PR changes the unmount handler default behavior. It no longer cleans up the mounted directories, unless the cleanupNodesOnUmount is explicitly set to true. This means that mounting something at /wordpress/wp-content/plugins/my-plugin will leave an empty directory in there after the unmount. This should not affect git status as git does not version empty directories.

If we really wanted to keep cleaning up the mount-related directories, we'd need to follow the following runtime rotation procedure:

1. Change the CWD to /
2. Unmount from old FS
3. Create new runtime and new FS
4. Mount to new FS
5. Try to restore the previous CWD
6. Only throw on failure at this point, not at the temporary unmount

So, effectively, move the failure if the path structure is off after the rotation, not during the rotation.

Testing Instructions (or ideally a Blueprint)

CI unit tests.

For manual testing, do this:

$ cli.ts server --mount-before-install=./test-site/:/wordpress

Then:

1. Let it do the setup.
2. Modify wp-content/themes/twentytwentyfour/functions.php to call non_existent_function(). This will nudge the PHP class to rotate the runtime and call unmount() for all the mounts,
3. Run the following command:

$ cli.ts server --mount-before-install=./test-site/:/wordpress  --skip-wordpress-setup

Finally, visit the local site. You should see an error screen. Refresh it three times, confirm you still see the same WordPress error screen.

[adamziel]

Unrelated, let's move it to another pr

[adamziel]

this is just code reformatting, I'll move this to another pr

[adamziel]

Let's ditch this approach and move forward with #2714 so we don't lose the "cleanup after ourselves" semantics of nodefs mounts.