(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/webpack.api.config.js

@@ -0,0 +1,44 @@
+const path = require('path');
+
+module.exports = {
+  target: 'web',
+  entry: {
+    "grist-plugin-api": "app/plugin/grist-plugin-api",
+  },
+  output: {
+    sourceMapFilename: "[file].map",
+    path: path.resolve("./static"),
+    library: "grist"
+  },
+  devtool: "source-map",
+  node: false,
+  resolve: {
+    extensions: ['.ts', '.js'],
+    modules: [
+      path.resolve('.'),
+      path.resolve('./ext'),
+      path.resolve('./stubs'),
+      path.resolve('./node_modules')
+    ],
+    fallback: {
+      'path': require.resolve("path-browserify"),
+    },
+  },
+  optimization: {
+    minimize: false, // keep class names in code
+  },
+  module: {
+    rules: [
+      {
+        test: /\.(js|ts)?$/,
+        loader: 'esbuild-loader',
+        options: {
+          loader: 'ts',
+          target: 'es2017',
+          sourcemap: true,
+        },
+        exclude: /node_modules/
+      },
+    ]
+  }
+};