refactor: use vim.g config and lazy loading

Replace setup() with vim.g.http_codes configuration. Add plugin/ file
with load guard for proper lazy loading via lazy.nvim or other managers.

plugin/http-codes.lua

@@ -0,0 +1,8 @@
+if vim.g.loaded_http_codes then
+  return
+end
+vim.g.loaded_http_codes = 1
+
+vim.api.nvim_create_user_command('HTTPCodes', function()
+  require('http-codes').pick()
+end, {})