Feat: add --fzf-command option to override FZF_DEFAULT_COMMAND

[alberti42]

Rationale of this PR

This change adds an optional --fzf-command argument to the fzf plugin. When provided as a string, the plugin sets FZF_DEFAULT_COMMAND for the spawned fzf process. This enables users to control the candidate list (e.g., fd -d=1) without fragile chain of piped commands, or custom argument parsing.

I also published a standalone plugin at fzf-plus.yazi so users can try it immediately while this PR is under review.

This PR solves

• Keeps the plugin minimal and aligned with fzf’s own configuration model based on FZF_DEFAULT_COMMAND.
• Works cleanly with Yazi’s argument parsing (no POSIX-style token splitting).
• Avoids brittle fallbacks and complex parsing.

Usage example

[[mgr.prepend_keymap]]
on   = "z"
run  = "plugin fzf -- --fzf-command='fd -t=d -d=1'"
desc = "Jump via fzf (limited depth, only directories)"

Notes

• Only string values are accepted; if --fzf-command is present but not a string, the plugin shows an error.
• Behavior is unchanged when the option is absent.
• When a selection exists, fzf still receives the selection via stdin (default command is ignored, as expected).