fix: prevent rebuild failures on *NIX with improved handling of symlinks/file

[EvanDarwin]

The source of this PR was from build failures I was getting related to linked files in dist/cloudflare/:

✗ Build failed in 816ms
[vike-cloudflare] EEXIST: file already exists, symlink '../../../client/category/reviews/index.html' -> '/Users/evan/Documents/Code/project/dist/cloudflare/category/reviews/index.html'
    at symlink (node:internal/fs/promises:1001:10)
    at symlinkOrCopy (file:///Users/evan/Documents/Code/project/node_modules/.pnpm/[email protected][email protected][email protected][email protected][email protected]__c2be31428ae5fe96f66c824fc430c401/node_modules/vike-cloudflare/dist/index.js:183:5)
... {
  errno: -17,
  code: 'PLUGIN_ERROR',
  syscall: 'symlink',
  path: '../../../client/category/reviews/index.html',
  dest: '/Users/evan/Documents/Code/project/dist/cloudflare/category/reviews/index.html',
  pluginCode: 'EEXIST',
  plugin: 'vike-cloudflare',
  hook: 'writeBundle'
}

Attempting to create a linked file at a path that already exists results in an error in *NIX environments.

This patch narrows the error case to only non-symlinked files (let's not unlink entire files and directories we're not sure about) so the user can intervene, while allowing the standard build process to succeed.

[EvanDarwin]

I realize now that this patch may only affect the 0.1.x branch, as the build process seems to have changed in 0.2.x (fixing the underlying issue as well).

However, I'd still appreciate getting this merged as it affects the version of vike-cloudflare that's shipped in the templates on vike.dev, even if it affects projects running outside Windows / CI environments, and also using SSG.

[magne4000]

@EvanDarwin I created a v0.1 for this purpose. Can you rebase your fix on this branch?