Add the Extension (custom node) filter again to both Search and Nodes windows

### What problem are you experiencing?

In a previous version of the node search, it was possible to filter for specific custom nodes, but for some reason this feature has been removed. It was very useful for finding a specific node within a node pack or simply for figuring out which nodes contained that custom extension.

Now I see there’s a categories pane on the left, but it’s not very helpful since the nodes are scattered across different categories.

Is it possible to add that filter back? Or is it already there? I can’t seem to add any filters other than those related to inputs and outputs.

### When does this problem occur?

- Open Search or Node window
- Try to filter per custom node Extension

### 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

_No response_

### Ideas for solutions (Optional)

_No response_

### Additional context

_No response_

Start by reproducing the missing filter in both the Search and Node windows, then inspect how the existing input and output filters are implemented. Done means users can filter by custom node Extension in both windows and locate nodes from a node pack or extension.

Written by the indexing model from the issue text.