refactor: organise TypeScript config and migrate helpers (#44747)

* feat: allow more 1000 steps to be created at once

* refactor: start migrating to typescript

* refactor: delete-step to ts

* refactor: migrated some helpers

* refactor: migrate create-empty-steps

* refactor: migrate create-step-between

* refactor: finish migrating to TS

* refactor: migrate tests

* fix: ensure mock.restore is done after each test

* fix: prevent double-tscing

* fix: repair the tests

* chore: use ts-node for scripts

We don't need the performance boost of incremental compilation and
ts-node is easier to work with

* refactor: consolidate tsconfigs

* refactor: replace gulp

* fix: use ts-node for build-curriculum

* fix: allow ts compilation of config

* feat: create and use create:config script

* fix: add /config to eslint projects

* fix: remove gulp script

tools/challenge-helper-scripts/helpers/get-existing-step-nums.ts

@@ -0,0 +1,30 @@
+import fs from 'fs';
+import path from 'path';
+
+// Generates an array with the output of processing filenames with an expected
+// format (`step-###.md`).
+// ['step-001.md', 'step-002.md'] => [1, 2]
+function getExistingStepNums(projectPath: string): number[] {
+  return fs.readdirSync(projectPath).reduce((stepNums, fileName) => {
+    if (
+      path.extname(fileName).toLowerCase() === '.md' &&
+      !fileName.endsWith('final.md')
+    ) {
+      const stepNumString = fileName.split('.')[0].split('-')[1];
+
+      if (!/^\d{3}$/.test(stepNumString)) {
+        throw (
+          `Step not created. File ${fileName} has a step number containing non-digits.` +
+          ' Please run reorder-steps script first.'
+        );
+      }
+
+      const stepNum = parseInt(stepNumString, 10);
+      stepNums.push(stepNum);
+    }
+
+    return stepNums;
+  }, [] as number[]);
+}
+
+export { getExistingStepNums };