Allow rebinding the confirm key for search and prompt inputs

[mokashang]

PR Description

The confirm action for the search/filter prompt and for text input prompts (e.g. when naming a stash or creating a new branch) was hard-coded to <enter> in #4860, on the assumption that nobody would want to remap it. As reported in #5510, that assumption was wrong: users who rebind keybinding.universal.confirm to something like <c-j> could no longer confirm those two prompts at all.

This adds two new keybindings, confirmSearch and confirmPrompt, following the existing convention of context-specific confirm keys (confirmMenu, confirmSuggestion, confirmInEditor). Both default to <enter>, so the behaviour is completely unchanged for users who don't remap them; users who do can now point them at whatever key they like.

Fixes #5510.

Testing: added an integration test (config/rebind_confirm_search_and_prompt) that rebinds both keys to <c-y> and verifies that a search and a text prompt can each be confirmed with the rebound key. I confirmed the test fails on master (both prompts ignore the rebound key) and passes with this change. The existing search/prompt integration tests still pass.

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 (no new user-facing strings; the prompt confirm keeps the existing Confirm label)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (keybindings are re-read via resetKeybindings on config reload, same as the other confirm keys)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc