Feature Request: add `accept` / `accepted` / `on_accept` event

### Checklist

- [x] I have read through the manual page (`man fzf`)
- [x] I have searched through the existing issues
- [x] For bug reports, I have checked if the bug is reproducible in the latest version of fzf

### Output of `fzf --version`

0.66.1

### OS

- [x] Linux
- [ ] macOS
- [ ] Windows
- [ ] Etc.

### Shell

- [x] bash
- [ ] zsh
- [ ] fish

### Problem / Steps to reproduce

The `become(...)` action is extremely useful for writing scripts, but it can be bound via `--bind` only to explicit key bindings or events.

This means that scripts that execute a command on the selected item must hard-code the `accept` key bindings, which might not comply with the user's `fzfrc` or `FZF_DEFAULT_OPTS`.

Having an `accept` / `accepted` / `on_accept` event, similar to `jump` / `jump-cancel` / `reload` / etc., would allow scripts to execute commands on the selection regardless of the user's key bindings for the `accept` action.

Example script:
```bash
# `enter` and `tab` are bound to `accept`
tmux ls -F "#S" | fzf --tmux \
--prompt='Session> ' \
--bind='enter:become(tmux switch -t ={})' \
--bind='tab:become(tmux switch -t ={})' \
--bind='jump:become(tmux switch -t ={})'
# This doesn't work:
# --bind='accept:become(tmux switch -t ={})
```

Start with fzf's --bind event handling and the become(...) action, using the existing jump, jump-cancel, and reload events as references. Determine how an accept-related event should interact with user-defined accept bindings and verify the example bash script works without hard-coded keys. Done means the selected item triggers become through the new event while existing bindings remain compatible.

Written by the indexing model from the issue text.