feat(search): add fuzzy search for keybindings by description

[MrZeLee]

Description

Adds a fuzzy keymap search to which-key: instead of drilling through the
popup, you can search for a keybinding by its description and execute it.

It walks which-key's existing per-mode keymap tree to collect every executable
keybinding for the current mode and buffer, then presents them through
vim.ui.select, so it integrates with whatever picker the user already has
(builtin, telescope, snacks, fzf-lua, dressing, …) with no hard dependency.
Each entry shows the formatted key sequence plus a description breadcrumb that
prepends parent group names (e.g. git → commit changes). Picking an entry
feeds its keys to execute the binding.

This keeps the implementation small and self-contained — a new
lua/which-key/search.lua module plus thin entry points; no changes to the
existing rendering/state code.

Usage

:WhichKeySearch        " current mode (defaults to normal)
:WhichKeySearch v      " a specific mode

require("which-key").search()

Changes

• lua/which-key/search.lua (new) — collect → format → vim.ui.select → execute.
• lua/which-key/init.lua — expose require("which-key").search(opts).
• lua/which-key/config.lua — register the :WhichKeySearch [mode] command.
• README.md — feature bullet and a "Search" section.

Related Issue(s)

• Closes feature: Fuzzy Search for Keybinds Through Descriptions #1052

Screenshots