First draft of an fully-in-browser code generation project

build the code generator using `expandToString`

.github/workflows/actions.yml

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+name: Build
+
+on:
+  push:
+    branches:
+      - '**'
+    tags-ignore:
+      - '**'
+  pull_request_target: # enables builds of PRs to branch 'master' from forked repos, with the workflow being taken from the target revision, i.e. the 'master' branch
+    branches:
+      - main
+  workflow_dispatch: # enables manual triggering
+
+jobs:
+  build:
+    # suppress triggering this job on 'synchronize' events of PR's (i.e. branch updates) within the same repo, as that is covered by 'push' already, but fire on updates of PRs coming from outside
+    if: github.event_name != 'pull_request_target' || github.event.action != 'synchronize' || github.event.pull_request.base.repo.id != github.event.pull_request.head.repo.id 
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    timeout-minutes: 20
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v3
+    - name: Use Node.js
+      uses: actions/setup-node@v3
+      with:
+        node-version: '16'
+    - name: Build
+      shell: bash
+      run: |
+        npm ci
+

.gitignore

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+node_modules
+public/*
+out/
+!index.html

README.md

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+<div id="langium-logo" align="center">
+  <a href="https://github.com/langium/langium">
+    <img alt="Langium Logo" width="500" src="https://user-images.githubusercontent.com/4377073/135283991-90ef7724-649d-440a-8720-df13c23bda82.png">
+  </a>
+  <h3>
+    Example application demonstrating a fully-in-browser DSL implementation including Code Generation based on Langium's arithmetics example DSL
+  </h3>
+</div>
+
+<div id="badges" align="center">
+
+  [![Build](https://github.com/langium/langium-in-browser-codegen-example/actions/workflows/actions.yml/badge.svg)](https://github.com/langium/langium-in-browser-codegen-example/actions/workflows/actions.yml)
+  [![Gitpod Ready-to-Code](https://img.shields.io/badge/Gitpod-ready--to--code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/langium/langium-in-browser-codegen-example/)
+
+</div>
+
+<hr>
+
+This example project demonstrates the usage of Langium in the browser without any backend.
+The language server is run in a web worker. It parses and validates the input text on change and — in this special case — sends a serialized snapshot of the abstract syntax tree (AST) to the browser's main thread, provided the text is free of errors. The code generation is than executed in the browser's main thread, and the obtained code is executed subsequently.  
+_Remark_: This setup well-suited for the sake of this demostration, but it is not common in production (yet 😁).
+
+## Get started
+
+Run
+```
+npm i
+npm run serve
+```
+and open http://localhost:3000 in your browser.
+Open the browser's developer tools, and put a break point at https://github.com/langium/langium-in-browser-codegen-example/blob/a2e590d9aeccf94178ebed83ab64ad40ebe1c635/src/arithmeticsEditor/index.ts#L44