Respect overwrite options when copying symlinks

[jakezwang]

Copying a symlink currently replaces an existing destination symlink even with overwrite: false or clobber: false. It also succeeds with overwrite: false, errorOnExist: true instead of reporting the existing destination.

Apply the existing overwrite checks in the symlink branches of copy and copySync, before unlinking the destination. Tests cover valid and broken destination links, both overwrite options, errorOnExist, default replacement and missing destinations.

The new tests reproduce 12 failures before the fix. npm test passes on Node 20.20.2, 22.23.2 and 24.4.1: lint, 755 unit tests and ESM tests. Eight existing tests remain pending on macOS; cross-device and other platform tests were not run locally.

[RyanZim]

Of note, fs.cp() has the same behavior, and we would like to stay in sync with that: #912 (comment). So this should be potentially raised with Node itself.

[jakezwang]

Understood. This was independently found edge-case behavior rather than a production incident. I have not opened an issue in Node, and I have not changed the proposed behavior further. Keeping fs-extra aligned with fs.cp() is a design constraint; this needs an upstream decision before treating the PR as ready to merge.