Nodes 2.0: add Menu subdir nesting

### Is there an existing issue for this?

- [x] I have searched the existing issues and checked the recent builds/commits

### What problem are you experiencing?

Nodes 2.0, apart from breaking a significant amount of legacy nodes and plugins, has also managed to achieve a particularly impressive milestone: **breaking rgthree’s node nesting feature**, which already worked perfectly fine before. A remarkable achievement, considering that rgthree’s implementation solved a real problem.

**One very obvious improvement would be proper menu nesting, and not only for LoRAs or Checkpoints.**
For example, this **_ComfyUI Styler_** node (which I expanded using AI) already shows nested menus thanks to rgthree’s implementation: simply including a / in the name attribute creates a new menu level. **It worked flawlessly (see screenshot), until Nodes 2.0 decided otherwise.**

Meanwhile, Nodes 2.0 offers us… what exactly?
Endless, flat lists of nodes that grow longer every week, plus a filter textbox that assumes users want to type every time they need something.

**News flash:** Users don’t want to type anything just because ComfyUI refuses to group items that clearly belong to the same category or folder. Browsing infinite unstructured lists when they could be logically grouped is not a “design choice”, it’s just bad UX.

**Given that Nodes 2.0 has already demonstrated its ability to break things that worked, maybe it would be worth considering adding a genuinely useful feature instead?**
Menu nesting already exists, already works, and is already proven in the ecosystem. Implementing something similar at the core level would immediately improve usability instead of regressing it.

### When does this problem occur?

- When working with checkpoints/loras/files in different folders
- When browsing options from nodes such as ComfyUI Styler or any node with long lists.

### How often do you encounter this problem?

Multiple times per day

### How much does this problem affect your workflow?

Significantly slows down my work

### Current workarounds

Use rgthree's node nesting feature + disabling Nodes 2.0

### Ideas for solutions (Optional)

**Add native submenu nesting**, not only for Nodes 2.0 but for the current, actually-working nodes 1.0 as well.

### Additional context

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-7597-Nodes-2-0-add-Menu-subdir-nesting-2cc6d73d3650817c93a4c7da2e0d788b) by [Unito](https://www.unito.io)