add devcontainer and update README for push-button dev environment

.devcontainer/devcontainer.json

@@ -0,0 +1,28 @@
+// For format details, see https://aka.ms/devcontainer.json. For config options, see the
+// README at: https://github.com/devcontainers/templates/tree/main/src/javascript-node
+{
+	"name": "Node.js",
+	// Or use a Dockerfile or Docker Compose file. More info: https://containers.dev/guide/dockerfile
+	"image": "mcr.microsoft.com/devcontainers/javascript-node:0-20",
+	"features": {
+		"ghcr.io/devcontainers/features/rust:1": {}
+	},
+
+	// Features to add to the dev container. More info: https://containers.dev/features.
+	// "features": {},
+
+	// Use 'forwardPorts' to make a list of ports inside the container available locally.
+	// "forwardPorts": [],
+
+	// Use 'postCreateCommand' to run commands after the container is created.
+	"postCreateCommand": "curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh && npm install && npm run compile-wasm && cp ./src/test/.testCode.doenet ./src/test/testCode.doenet",
+	"postAttachCommand": {
+	  "viewer": "npm run dev -- --host"
+	}
+
+	// Configure tool-specific properties.
+	// "customizations": {},
+
+	// Uncomment to connect as root instead. More info: https://aka.ms/dev-containers-non-root.
+	// "remoteUser": "root"
+}

README.md

@@ -17,6 +17,11 @@ Semantic markup for building interactive web activities.
 
 
 ## Quickstart
+
+First, install [Node](https://nodejs.org/en/download),
+[Rust](https://rustup.rs/),
+and [wasm-pack](https://rustwasm.github.io/wasm-pack/installer/).
+
 In the project folder:
 
 `$ npm install`
@@ -25,9 +30,17 @@ In the project folder:
 
 `$ npm run dev`
 
-Paste demo code into `src/test/testCode.doenet`
+Paste demo code into `./src/test/testCode.doenet`
+(or copy `./src/test/.testCode.doenet` to `./src/test/testCode.doenet`).
+
+Navigate to <http://localhost:3000/src/test/index.html>
+
+## Codespaces quickstart
 
-Navigate to `localhost:3000/src/test/index.html`
+All the above steps should happen automatically. Once that is ready
+(after "Your applicaiton running on port 3000 is available"),
+just visit <http://localhost:3000/src/test/index.html> (may need to
+Ctrl-click to open GitHub Codespaces link).
 
 ## Demos
 

src/test/.testCode.doenet

@@ -0,0 +1,7 @@
+<graph name="graph">
+	<point name="p1" xs="2 3"/>
+	<point name="p2" xs="$p1.y $p1.x"/>
+</graph>
+<text>
+	<collect source="graph" componentType="point"/>
+</text>