(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

buildtools/build.sh

@@ -10,6 +10,8 @@ fi
 
 set -x
 tsc --build $PROJECT
+buildtools/update_type_info.sh app
 webpack --config buildtools/webpack.config.js --mode production
 webpack --config buildtools/webpack.check.js --mode production
+webpack --config buildtools/webpack.api.config.js --mode production
 cat app/client/*.css app/client/*/*.css > static/bundle.css