(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

.gitignore

@@ -2,6 +2,7 @@
 /_build/
 /static/*.bundle.js
 /static/*.bundle.js.map
+/static/grist-plugin-api*
 /static/bundle.css
 /static/browser-check.js
 /static/*.bundle.js.*.txt