linting: disallow t.identifier("undefined") in plugins (#6096)

* add new custom eslint rule, replace remaining t.identifier("undefined") with buildUndefinedNode(), update tests

* change no-undefined-identifier reporting descriptor

packages/babel-plugin-transform-es2015-function-name/test/fixtures/function-name/modules-3/expected.js

@@ -3,7 +3,7 @@
 Object.defineProperty(exports, "__esModule", {
   value: true
 });
-exports.default = undefined;
+exports.default = void 0;
 
 var _store = require("./store");