Suggestions: Add scroll back as suggestion source.

Nobody has claimed this yet.

### Description of the new feature

This is based on Vim's out of the box completions where it sources the options from words in the open buffers. In this case it would pull the options from the active terminal buffer.

The case where I have found this really useful is when I have run a command that returns some data and now want to use part of the result in my next command. Ex. After running git log, running an additional command on one of the hashes and being able to auto complete it. Or After running --help on a command and then being able to auto complete the options.

Being able to define a regex for the filter was really useful to be able to have targeted completions for common stuff (hashes, addresses, command line flags) and then a catch all "default" one for words with > 4 chars.

Example Config.
```json
{
"command":
{
"action": "showSuggestions",
"source": "scrollback",
"regex": "[^\\s]{5,}"
},
"id": "User.showSuggestions.2F7A7303"
},
{
"command":
{
"action": "showSuggestions",
"source": "scrollback",
"regex": "-{1,2}\\w(?:[\\w+|-])+"
},
"id": "User.showSuggestions.2F7A7304"
},
{
"command":
{
"action": "showSuggestions",
"source": "scrollback",
"regex": "\\b(?:0x)?[0-9A-Fa-f]{8,16}(?:`[0-9A-Fa-f]{1,16})?|\\b[0-9A-Fa-f]{7,40}\\b"
},
"id": "User.showSuggestions.2F7A7305"
},
```
This what it looked like when I was testing what I had hacked together locally.

https://github.com/user-attachments/assets/7719493f-a5f0-43c2-a4f1-bfb4505d8e86

### Proposed technical implementation details

_No response_

Start by reviewing the existing showSuggestions command and its configuration handling, then trace how terminal scrollback is exposed. Define how the scrollback source and optional regex should behave, using the provided examples as acceptance cases; the feature is done when those configured suggestion sources work in the terminal.

Written by the indexing model from the issue text.