Add terminal grouping support

[Lancelotbronner]

Description

This is still a draft, it's (mostly) working but needs polish and bugfixing. Here are the refactors & features:

• Refactored terminal state from being spread out over a few views/models to being aggregated into a single TerminalEmulator model, simplified a few views (this was required for multiple simultaneous shells)
• Added a TerminalGroup model to represented grouped terminals
• Drag & drop one or more terminals onto an ungrouped one to create a group
• Drag & drop one or more terminals onto a group to move them into it
• Freely rename groups and terminals (or leave them automatic by removing your custom title)

Related Issues

• #379

Checklist

• [ ] Bugfix: moving sometimes flattens the disclosure groups into the list (SwiftUI???)
• [ ] Cleanup the models (I'd like some feedback on it first)
• [ ] Let groups be part of drag & drop operations
• [ ] Ensure each terminals get their own "mini-toolbar"
• [ ] Cleanup the views (I know its hacked together, I'd like to have the model feedback first)

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• [ ] Wishlist: insert terminals at a specific spot between sibling groups (maybe a SwiftUI limitation)
• [ ] Wishlist: click to focus in the terminal area (maybe a SwiftTerm limitation)

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• [ ] I read and understood the contributing guide as well as the code of conduct
• [ ] The issues this PR addresses are related to each other
• [ ] My changes generate no new warnings
• [ ] My code builds and runs on my machine
• [ ] My changes are all related to the related issue above
• [ ] I documented my code

Screenshots

[tom-ludwig]

Just wanted to throw this gem in here, I found it in Figma a few weeks ago and almost forgot to share it:

[austincondiff]

@activcoding Yeah, that was definitely an experimental design. There are a few concerns I had with it so I ended up moving on.

[Lancelotbronner]

Personally I don't really like it so I'm glad it's gone but ~~that does give me an idea~~.

~~I'm having issues with DisclosureGroup messing with onMove indexes and preventing it to work as intended and it might be fixable by switching the design a bit.~~

~~I'll implement it (a List using Section for groups) and post a screenshot back see what people think of it.~~

Update: It doesn't work, turns out it's ForEach that messes with the indexes, I'm not sure what the solution is but it would also improve the behaviour of the files outline on the sidebar.

Maybe we should file a radar for OutlineGroup to conform to DynamicViewContent.