feat: add <Plug> mappings (#27)

## Problem

Users who want keybindings must call Lua functions directly or wrap
commands in closures. There is no stable public API for key binding.

## Solution

Define <Plug> mappings in the plugin file and document them in a new
MAPPINGS section in the vimdoc.

doc/live-server.txt

@@ -48,6 +48,27 @@ COMMANDS                                               *live-server-commands*
 :LiveServerToggle [dir]   Toggle the live server on or off. If `dir` is
                           provided, toggles the server in that directory.
 
+===============================================================================
+MAPPINGS                                               *live-server-mappings*
+
+                                                    *<Plug>(live-server-start)*
+<Plug>(live-server-start)   Start the live server. Equivalent to
+                            |:LiveServerStart| with no arguments.
+
+                                                     *<Plug>(live-server-stop)*
+<Plug>(live-server-stop)    Stop the live server. Equivalent to
+                            |:LiveServerStop| with no arguments.
+
+                                                   *<Plug>(live-server-toggle)*
+<Plug>(live-server-toggle)  Toggle the live server. Equivalent to
+                            |:LiveServerToggle| with no arguments.
+
+Example configuration: >lua
+    vim.keymap.set('n', '<leader>ls', '<Plug>(live-server-start)')
+    vim.keymap.set('n', '<leader>lx', '<Plug>(live-server-stop)')
+    vim.keymap.set('n', '<leader>lt', '<Plug>(live-server-toggle)')
+<
+
 ===============================================================================
 API                                                        *live-server-api*
 
@@ -87,9 +108,7 @@ Start server in project root: >vim
 <
 Lua keybinding to toggle server: >lua
 
-    vim.keymap.set('n', '<leader>ls', function()
+    vim.keymap.set('n', '<leader>ls', '<Plug>(live-server-toggle)')
-        require('live-server').toggle()
-    end)
 <
 Configuration with custom port and no auto-open: >lua
 

plugin/live-server.lua

@@ -14,3 +14,13 @@ end, { nargs = '?' })
 vim.api.nvim_create_user_command('LiveServerToggle', function(opts)
   require('live-server').toggle(opts.args)
 end, { nargs = '?' })
+
+vim.keymap.set('n', '<Plug>(live-server-start)', function()
+  require('live-server').start()
+end, { desc = 'Start live server' })
+vim.keymap.set('n', '<Plug>(live-server-stop)', function()
+  require('live-server').stop()
+end, { desc = 'Stop live server' })
+vim.keymap.set('n', '<Plug>(live-server-toggle)', function()
+  require('live-server').toggle()
+end, { desc = 'Toggle live server' })