Adding "custom widget builder" widget (#141)

Simple widget editor (based on monaco editor), that can edit and serve widgets right from the Grist document.

PR:
#141

README.md

@@ -41,3 +41,16 @@ Next start Grist with an URL pointing to a local widget manifest file:
 ```bash
 GRIST_WIDGET_LIST_URL=http://localhost:8585/manifest.json npm start
 ```
+
+Alternatively you can run the widget repository development server alongside with the Grist docker image preconfigured to use it:
+
+```bash
+yarn run grist:serve
+```
+
+or run it in development mode with automatic reload:
+
+```bash
+yarn install
+yarn run grist:dev
+```

custom-widget-builder/README.md

@@ -0,0 +1,38 @@
+# Grist Custom Widget
+
+This widget enables you to create other custom widgets for Grist documents, right inside Grist that are hosted by Grist itself.
+
+## Getting Started
+
+To begin developing your custom widget, follow these steps:
+
+1. **Open the Widget Editor:** Click on the "Open configuration" button in the creator panel or clear the saved filter settings for the relevant tab.
+2. **Edit Code:** Write your widget's logic in the JavaScript tab and structure its appearance in the HTML tab.
+3. **Preview and Install:** Click the "Preview" button to see your widget in action. This will save the widget's code to the document's metadata.
+4. **Save Configuration:** Press the "Save" button to persist the widget settings to ensure they remain active after refreshing the page.
+
+**Note:** There is no autosave feature, so always remember to save your configuration manually.
+
+## Data Storage
+
+The widget's configuration data is stored in the widget's metadata using the following format:
+
+```javascript
+const options = {
+    _installed: true,
+    _js: `...your JavaScript code...`,
+    _html: `...your HTML code...`,
+};
+grist.setOptions(options);
+```
+
+In the final widget, the <code>_html</code> field is inserted as is into an iframe, and the <code>_js</code> field is embedded within a script tag afterwards.
+
+This widget in itself doesn't require any access to documents metadata, but it can be used to create widgets that do. Storing <code>Javascript</code> and <code>HTML</code> code in the metadata stores it only temporarily. User needs to save it in order to persist the changes (just like for regular <code>filters</code>).
+
+Any contribution is welcome, the big thing missing is dark mode support.
+
+
+## IntelliSense
+
+The widget editor supports `IntelliSense` for the `JavaScript` code. It does it by providing its own types definitions directly to the Monaco editor. The IntelliSense is based on the official Grist Plugin API. See the `genarate.js` script for more details.