Add transient menu

[rogsme]

Closes #341

Here's the new transient menu!

How it looks outside the chatgpt-shell buffer:

How it looks in the chatgpt-shell buffer (C-c C-t by default):

How it looks on region selection:

Some videos:

General demo:

demo.mp4

Region selection demo:

highlight-demo.mp4

[xenodium]

Hi @rogsme!

Thanks a lot for the PR! It'll be super handy to have these menus.

While I haven't done a thorough review, maybe we should iterate on the menu organization, naming, bindings... you know thet stuff we all love to bikeshed on :)

You have a really great selection of commands. I'm thinking we can likely consolidate some sections a little and maybe tweak some names...

Menu organization

The following is only a rough brain dump (and totally open for discussion), there's no need to implement just yet. The idea is to pin down general menu organization and once we have that, we can go and implement.

Shells                     Compose prompt via                Inline edit

b Switch to shell buffer   e Dedicated buffer                q Quick insert/edit
N Create new shell         p Minibuffer
                           P Minibuffer (include last kill)

Session               History     Navigation              Source blocks    Transcript

m Swap model          ? Next      j Next item             x Execute        S Save
L Reload models       ? Previous  k Previous item         E Edit           O Restore
m Swap system prompt  ? Search    J Next source block     V View
                                  K Previous source block

Bindings

We have options here:

1. Reuse existing bindings: Something to consider here is that by reusing the existing bindings, the transient menu becomes a "help sheet" of sorts and trains users over time.
2. New bindings (reuse when possible): We use single char bindings, but try to relate them to existing bindings when possible. For example Next/Previous are TAB/Backtab (unless we go for idiomatic n/p, more on that below).

Vanilla vs Evil

If possible, let's try to use idiomatic bindings when possible. For example, n/p are prevalent in Emacs for moving next/previous (gotta say though TAB is pretty handy). Executing org blocks is well known to be C-c C-c, etc.

We could also make it smart to use vanilla vs evil.

Stickiness

I think this is possible with transient, but I haven't looked, maybe making the menu sticky for certain commands that are meant to be repeated? For example next/previous.

These are just thoughts for now. Lemme know what you think.