[enhancement]: Changing the node system and support for extensions

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Contact Details

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### What should this feature add?

## Nodes

I propose to add an additional tab to txt2img, img2img and canvas modes instead of the current node mode, when switching to it, the user gets to the field with configured nodes of the current mode, which he can change or refine. It doesn't specify parameters in the nodes themselves, but takes those parameters from their input nodes, where those parameters fall into from the UI.

## Extensions

Since InvokeAI does not use Gradio, I suggest to divide extensions into Frontend and Backend as it is done in EasyDiffusion.

### Frontend extensions

These are .js files that are loaded into the browser along with the rest of the web interface. They can be used to modify the interface or implement various features that do not require backend modifications, such as translating a prompt.

### Backend extensions

These are folders with various Python scripts that directly affect the backend. They can also contain a js script to extend the web interface. Unlike nodes, extensions are not limited to generation, for example they can be used to add a LoRA downloader from civitai.com, or support for other mods such as Kandinsky.

### Alternatives

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### Additional Content

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