feat: Add alternative key for return

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PR Description

This PR added an optional alternative key (return-alt) for return, similar to quit-alt1.

The motivation of doing this is that personally I prefer use l for enter and h for back, which works fine if i set return to h in the config. But in some input window such as commit, h will be failed. So by adding this alternative key, I can have both Esc and h for return.

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)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc