[new-platform] create base new platform bundle

[spalger]

Part of #16253

Until we get #10908 we want a solution for working and shipping new-platform stuff in the UI. To get started we will create a new bundle in the optimizer dedicated for the new platform. This bundle will be loaded any time a uiExports.app is not found for an application, allowing it to find the application in the newPlatform bundle or render a 404 page.

With this mechanism the server doesn't need to know about any plugin-provided apps, "legacy" applications can continue to exist in their current form, and we can start figuring out more new-platform stuff.

• newPlatform bundle in the optimizer
• basic chrome rendered in the page
• renders a 404 because there are no applications at this time (See [browser] Application service #18843)

[spalger]

Splitting up into

1. [new-platform] bootstrap the legacy platform within the new platform [new-platform] bootstrap the legacy platform within the new platform #20694
2. [new-platform] migrate fatal error page to new platform [new-platform] migrate fatal error page to new platform #20695
3. [new-platform] migrate toast notification system to new platform [new-platform] migrate toast notification system to new platform #20698
4. [new-platform] migrate UiSettings client to new platform [new-platform] migrate UiSettings client to new platform #20697
5. [new-platform] migrate ui/chrome to new platform [new-platform] migrate ui/chrome APIs to new platform #20696