Add browser extension template (huggingface#23)

* [WIP] Browser extension template

* Prevent concurrent WebGPU EP execution

Needed since WebGPU EP only supports one session execution at a time

* Update comments

* Update webpack.config.js

* Use q4 model by default

Author: xenova

browser-extension/.gitignore

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+node_modules/

browser-extension/README.md

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+# Transformers.js - Sample browser extension
+
+An example project to show how to run 🤗 Transformers in a browser extension. Although we only provide instructions for running in Chrome, it should be similar for other browsers.
+
+## Getting Started
+
+1. Clone the repo and enter the project directory:
+   ```bash
+   git clone https://github.com/huggingface/transformers.js-examples.git
+   cd transformers.js-examples/browser-extension/
+   ```
+1. Install the necessary dependencies:
+
+   ```bash
+   npm install
+   ```
+
+1. Build the project:
+
+   ```bash
+   npm run build
+   ```
+
+1. Add the extension to your browser. To do this, go to `chrome://extensions/`, enable developer mode (top right), and click "Load unpacked". Select the `build` directory from the dialog which appears and click "Select Folder".
+
+1. That's it! You should now be able to open the extension's popup and use the model in your browser!
+
+## Editing the template
+
+We recommend running `npm run dev` while editing the template as it will rebuild the project when changes are made.
+
+All source code can be found in the `./src/` directory:
+
+- `background.js` ([service worker](https://developer.chrome.com/docs/extensions/mv3/service_workers/)) - handles all the requests from the UI, does processing in the background, then returns the result. You will need to reload the extension (by visiting `chrome://extensions/` and clicking the refresh button) after editing this file for changes to be visible in the extension.
+
+- `content.js` ([content script](https://developer.chrome.com/docs/extensions/mv3/content_scripts/)) - contains the code which is injected into every page the user visits. You can use the `sendMessage` api to make requests to the background script. Similarly, you will need to reload the extension after editing this file for changes to be visible in the extension.
+
+- `popup.html`, `popup.css`, `popup.js` ([toolbar action](https://developer.chrome.com/docs/extensions/reference/action/)) - contains the code for the popup which is visible to the user when they click the extension's icon from the extensions bar. For development, we recommend opening the `popup.html` file in its own tab by visiting `chrome-extension://<ext_id>/popup.html` (remember to replace `<ext_id>` with the extension's ID). You will need to refresh the page while you develop to see the changes you make.

browser-extension/build/.gitignore

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+# Running `npm run build` will build the project and output the files here.
+*
+!.gitignore