Preserve symlinks when copying folders or files

Today our [`copy`](https://github.com/microsoft/vscode/blob/da0a04fffb90b0f4365b038c39c9e78d97d64e6b/src/vs/base/node/pfs.ts#L512-L512) function does not preserve symbolic links but simply copies the target over. We should probably try to preserve symlinks if possible (e.g. only on POSIX OS).