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.
