(core) Moving widget tests to core

Summary:
- Custom widget tests are now in grist-core
- Adding buildtools for grist-plugin-api.js

Test Plan: Existing tests

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3617

package.json

@@ -81,6 +81,7 @@
     "sinon": "7.1.1",
     "source-map-loader": "^0.2.4",
     "tmp-promise": "1.0.5",
+    "ts-interface-builder": "0.3.2",
     "typescript": "4.7.4",
     "webpack": "5.73.0",
     "webpack-cli": "4.10.0",