Support structured "browser" property in package.json

Today VS Code supports a `browser` property of type string in the `package.json` to specify an extension's entry point for web worker based extensions. When using `esbuild` for bundling the extension, the `browser` property of the package.json can be used to substitute some node-only built-in packages for a browser-friendly alternative by listing (see https://esbuild.github.io/getting-started/#bundling-for-the-browser). When this esbuild feature is used, VS Code runs into an issue and shows this alert: <img width="483" alt="2021-11-02_17-12-15" src="https://user-images.githubusercontent.com/1898161/140031504-a7618840-91eb-4e4f-9690-325f17f34bdb.png"> I suggest that VS Code accepts a structured `browser` property and looks for a `main` sub-property in this case. /cc @jrieken @alexdima @sandy081