update validatem to @validatem/core

src/postcss/postcss-icss-parser.js

@@ -14,7 +14,9 @@ Licensed under:
 const postcss = require("postcss");
 const icssUtils = require("icss-utils");
 const loaderUtils = require("loader-utils");
-const { validateOptions, required, isFunction } = require("validatem");
+const { validateOptions } = require("@validatem/core");
+const required = require("@validatem/required");
+const isFunction = require("@validatem/is-function");
 
 const pluginName = 'postcss-icss-parser';