fix(node): some directory separators are still inconsistent between p…

…latforms in tasks.json and package.json (projen#3406)

Related to projen#3284 (comment)

This is the second attempt at fixing the directory separator issue for lambda entrypoints, which was started in projen#3387. 

I tried adding unit tests for `renderBundleName`. Unfortunately, this can't be unit-tested now because NodeJS exports the `path` package for the Linux platform instead of Windows at import time and not at runtime.
https://github.com/nodejs/node/blob/f28ccd3941e70a825786eff7e28f59014ac76ac9/lib/path.js#L1546

I tested it locally in an example project, and it looks like it's fixed now.
![image](https://github.com/projen/projen/assets/22875166/4f98fb92-0a45-4d83-845c-152e48ded763)


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src/javascript/util.ts

@@ -27,7 +27,7 @@ export function renderBundleName(entrypoint: string) {
     parts.shift(); // just remove 'src' if its the first element for ergonomics
   }
 
-  const p = parts.join(sep);
+  const p = parts.join(posix.sep);
   const dir = dirname(p);
   const base = basename(p, extname(p));
   return posix.join(dir, base);