plugins/utility/fff-nvim: add fff.nvim module

Adds fff.nvim, a high-performance Rust-backed file picker for Neovim
with fuzzy search and frecency scoring. The Rust shared library is
built from source via rustPlatform.buildRustPackage and symlinked into
target/release/ where the plugin's lua loader expects it.

Toggle with vim.utility.fff-nvim.enable; pass arbitrary fff.setup()
options through vim.utility.fff-nvim.setupOpts.

modules/plugins/utility/default.nix

@@ -4,6 +4,7 @@
     ./ccc
     ./diffview
     ./direnv
+    ./fff-nvim
     ./fzf-lua
     ./gestures
     ./harpoon