Add return-alt1 to allow for a secondary Return key

[JelteF]

• PR Description

This partially adds the feature back that was removed in #2495. This
allows specifing a secondary Return keybinding. In my personal setup I
use the q for this (and remove it as the default for Quit). Returning
is a very common operation in lazygit, and having to reach for the
Escape key all the time is annoying. There are cases where it's not
possible to use a regular letter as the Return keybinding, such as the
commit prompt.

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc