Add Squash merge (#3566)

- **PR Description**

Hello,

This PR add merge --squash. A PR already exist
https://github.com/jesseduffield/lazygit/pull/3130, but the author
abandoned it, so I remake it.
I modified to fit most of the comment made except this one
https://github.com/jesseduffield/lazygit/pull/3130/files#r1404808121. I
didn't find an existing example and thus didn't know to modify the code
to fit it to the new way of doing things.

There's still the choice box to commit or not to do as discussed
https://github.com/jesseduffield/lazygit/pull/3130#issuecomment-2112324990.
I'll do it when I have time, I first need to read the code to see how it
really works.

Also only english has been made for now. 


![image](https://github.com/jesseduffield/lazygit/assets/94681915/f648ca13-3d16-4703-a074-a83fe9a1eb0f)


- **Please check if the PR fulfills these requirements**

* [x] Cheatsheets are up-to-date (run `go generate ./...`)
* [x] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [x] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [x] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [x] Docs (specifically `docs/Config.md`) have been updated if
necessary
* [x] You've read through your own file changes for silly mistakes etc

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