Move extension to webpack-based build to support Typescript sources

[dfederschmidt]

This PR moves the build / development process to support Typescript files.

For now, all JS will work as-is since all valid JavaScript is also valid TypeScript, enabling an incremental adoption of Typescript. But since we need to compile TS sources into JS at build time, webpack is introduced.

Currently, the main entrypoint to the extension is out/extension.js. With webpack added, the npm run watch task will use webpack to compile all sources inside out/ into the dist/ folder. Now, dist/extension.ts is the new entrypoint for the extension.

The PR changes the "Run Extension" debug task to automatically launch the npm run watch task to ensure that webpack is run continuously throughout the development.

It also adds a new debug task to run the extension in an isolated extension host with a temporary settings profile ("Run Extension Isolated"). Note that this requires workbench.experimental.settings profiles.enabled to be set to true in the developers user settings.