refactor: Remove last remaining circular import in core/ (#6818)

* refactor(xml): Move textToDom to core/utils/xml.ts

  This function being in core/xml.ts was the cause for the last
  remaining circular import in core/ (between variables.ts and
  xml.ts).

  Moving it to utils/xml.ts makes sense anyway, since there is
  nothing Blockly-specific about this function.

  Fixes #6817.

* fix(closure): Reenable goog.declareModuleId multiple-call check

  Reenable an assertion which check to make sure that
  goog.declareModuleId is not called more than once in a module
  (and which also catches circular imports amongst ES modules, which
  are not detected by closure-make-deps).

* chore(tests,demos): Augo-migrate use of textToDom

  Testing the migration file entry by auto-migrating all uses of
  Blockly.Xml.textToDom to Blockly.utils.xml.textToDom.

* chore(blocks): Manually migrate remaining use of textToDom

  Update the one remaining call to textToDom (in blocks/lists.ts)
  to the function's new location - also removing the last use of
  the Blockly.Xml / core/xml.ts) module from this file.

* docs(xml): Remove unneeded @alias per comments on PR #6818

* fix(imports): Remove unused import

tests/mocha/serializer_test.js

@@ -1830,7 +1830,7 @@ const runSerializerTestSuite = (serializer, deserializer, testSuite) => {
   const createTestFunction = function(test) {
     return function() {
       Blockly.Xml.domToWorkspace(
-          Blockly.Xml.textToDom(test.xml), this.workspace);
+          Blockly.utils.xml.textToDom(test.xml), this.workspace);
       if (serializer && deserializer) {
         const save = serializer(workspaces.save(this.workspace));
         this.workspace.clear();