(core) Move report-why-tests-hang helper to core

Summary:
This helps forcibly end mocha tests when they hang, and print out
something that may help debug the situation.

Also add the generated static/bundle.css file to core/.gitignore.

Also, avoid using npm-packages-offline-cache when building core, by avoiding use of .yarnrc which turns it on.

Test Plan: Tested manually

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2788

package.json

@@ -60,7 +60,8 @@
     "stats-webpack-plugin": "^0.7.0",
     "typescript": "3.9.3",
     "webpack": "4.41.0",
-    "webpack-cli": "3.3.2"
+    "webpack-cli": "3.3.2",
+    "why-is-node-running": "2.0.3"
   },
   "dependencies": {
     "@gristlabs/connect-sqlite3": "0.9.11",