feat: add <Plug> mappings

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/http-codes.txt

@@ -16,6 +16,17 @@ USAGE                                                            *:HTTPCodes*
     :HTTPCodes
 <
 
+===============================================================================
+MAPPINGS                                                *http-codes-mappings*
+
+                                                      *<Plug>(http-codes-pick)*
+<Plug>(http-codes-pick)     Open the HTTP status code picker.
+                            Equivalent to |:HTTPCodes|.
+
+Example configuration: >lua
+    vim.keymap.set('n', '<leader>hc', '<Plug>(http-codes-pick)')
+<
+
 ===============================================================================
 CONFIGURATION                                             *vim.g.http_codes*
 

plugin/http-codes.lua

@@ -6,3 +6,5 @@ vim.g.loaded_http_codes = 1
 vim.api.nvim_create_user_command('HTTPCodes', function()
   require('http-codes').pick()
 end, {})
+
+vim.keymap.set('n', '<Plug>(http-codes-pick)', function() require('http-codes').pick() end, { desc = 'Pick HTTP status code' })