feat: add various acceptance keys (#2928)

This change introduces (optional) acceptance keys of Backspace and Left
Arrow, when at the start of a line. These two are common muscle memory
actions for users.

The configuration defaults to false so as not to disrupt existing user
patterns.

This also adds a test that exercises the various acceptance modes, which
as it turns out was quite easy to do.

I discussed this on discord where [Ellie suggested I raised an
issue](https://discord.com/channels/954121165239115808/1421180955657244703/1422642337481228400),
but I felt like a PR would be more tangiable. I've tested this locally
and I'm very happy with how these keys work, it fits my needs well.

`exit_past_line_start` and `accept_past_line_start` can technically
co-exist. When this happens `accept_past_line_start` takes precedence.
Is this okay, or should we reconsider the config? Perhaps
`acceptance_keys = []` would be better here? I'm very open to changes
here.

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## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
- [x] I have checked that there are no existing pull requests for the
same thing

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