fix: add deprecated setup() for backward compatibility

Restores setup() with deprecation warning via vim.deprecate(). Users can
continue using setup() while migrating to vim.g.http_codes.

lua/http-codes.lua

@@ -68,4 +68,15 @@ end
 
 M.http_codes = M.pick
 
+---@deprecated Use `vim.g.http_codes` instead
+function M.setup(user_config)
+  vim.deprecate('require("http-codes").setup()', 'vim.g.http_codes', '2.0.0', 'http-codes.nvim')
+
+  if user_config then
+    vim.g.http_codes = vim.tbl_deep_extend('force', vim.g.http_codes or {}, user_config)
+  end
+
+  init()
+end
+
 return M