Make `smart-enter.yazi` support integration with the builtin `fzf` plugin

[sxyazi]

Add a new integrations configuration to smart-enter.yazi, such as:

require("smart-enter"):setup {
  integrations = {
    fzf = true,
  }
}

Once enabled (fzf = true), the behavior of the built-in fzf will change: for files, open; for directories, enter.

This requires the fzf plugin to emit the user's selection as an event via DDS, so that other plugins subscribing to this event can intercept and implement custom behavior.