Proposed API sample. Fix microsoft/vscode#33358

Author: octref

proposed-api-sample/.gitignore

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+out
+node_modules
+.vscode-test/
+*.vsix

proposed-api-sample/.vscode/launch.json

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+// A launch configuration that compiles the extension and then opens it inside a new window
+// Use IntelliSense to learn about possible attributes.
+// Hover to view descriptions of existing attributes.
+// For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+{
+	"version": "0.2.0",
+	"configurations": [{
+			"name": "Run Extension",
+			"type": "extensionHost",
+			"request": "launch",
+			"runtimeExecutable": "${execPath}",
+			"args": [
+				// "--enable-proposed-api",
+				"--extensionDevelopmentPath=${workspaceFolder}"
+			],
+			"outFiles": [
+				"${workspaceFolder}/out/**/*.js"
+			],
+			"preLaunchTask": "npm: watch"
+		}
+	]
+}

proposed-api-sample/.vscode/tasks.json

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+// See https://go.microsoft.com/fwlink/?LinkId=733558
+// for the documentation about the tasks.json format
+{
+	"version": "2.0.0",
+	"tasks": [
+		{
+			"type": "npm",
+			"script": "watch",
+			"problemMatcher": "$tsc-watch",
+			"isBackground": true,
+			"presentation": {
+				"reveal": "never"
+			},
+			"group": {
+				"kind": "build",
+				"isDefault": true
+			}
+		}
+	]
+}

proposed-api-sample/README.md

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+# Proposed API Sample
+
+This is a sample illustrating how to use [Proposed API](https://vscode-ext-docs.azurewebsites.net/api/advanced-topics/using-proposed-api) in VS Code. It is adapted from the [Hello Code Sample](/hellocode-sample).
+
+The proposed API being used is `vscode.env.clipboard.readText()`. See details here: https://github.com/Microsoft/vscode/issues/217#issuecomment-433963587.
+
+## VS Code API
+
+### Proposed API
+
+- [`env.clipboard`](https://github.com/Microsoft/vscode/blob/master/src/vs/vscode.proposed.d.ts#L49)
+- [`Clipboard.readText`](https://github.com/Microsoft/vscode/blob/master/src/vs/vscode.proposed.d.ts#L44)
+
+## Running the Sample
+
+This sample runs on VS Code Insider with commit [4626bc0e1b679d555dd6f034ece35ffc394b3bb7](https://github.com/Microsoft/vscode/commit/4626bc0e1b679d555dd6f034ece35ffc394b3bb7). If VS Code updated the Clipboard API in the future, you must re-download [`vscode.proposed.d.ts`](https://github.com/Microsoft/vscode/blob/master/src/vs/vscode.proposed.d.ts) to `src/vscode.proposed.d.ts` and update your API usage accordingly.
+
+- Run `npm install` in terminal to install dependencies
+- Run the `Run Extension` target in the Debug View. This will:
+	- Start a task `npm: watch` to compile the code
+	- Run the extension in a new VS Code window