fix: Continue setup even if a file to delete is not found (#663)

* Continue setup even if a file to delete is not found

For various reasons, `.gitkeep` may be deleted already.

(In my case, even though I followed the [Getting Started](https://quartz.jzhao.xyz) instructions exactly, my first run resulted in an `fatal: 'upstream' does not appear to be a git repository`)

If we try to delete `.gitkeep` again and don't ignore `ENOENT`, then the whole setup fails.

* Use fs.existsSync

quartz/cli/handlers.js

@@ -113,7 +113,10 @@ export async function handleCreate(argv) {
     }
   }
 
-  await fs.promises.unlink(path.join(contentFolder, ".gitkeep"))
+  const gitkeepPath = path.join(contentFolder, ".gitkeep")
+  if (fs.existsSync(gitkeepPath)) {
+    await fs.promises.unlink(gitkeepPath)
+  }
   if (setupStrategy === "copy" || setupStrategy === "symlink") {
     let originalFolder = sourceDirectory