New platform foundation in the UI

[kimjoar]

[initial meta issue for UI work related to new-platform, more details to come]

• [new-platform] migrate ui/chrome APIs to new platform #20696 [new-platform] migrate ui/chrome to new platform
  • [new-platform] bootstrap the legacy platform within the new platform #20694 [new-platform] bootstrap the legacy platform within the new platform
  • [new-platform] migrate fatal error page to new platform #20695 [new-platform] migrate fatal error page to new platform
  • [new-platform] migrate toast notification system to new platform #20698 [new-platform] migrate toast notification system to new platform
  • [new-platform] migrate UiSettings client to new platform #20697 [new-platform] migrate UiSettings client to new platform
• [browser] Application service #18843 [new-platform] Application service (include statusPage app)
• Directory structure with the new platform #12466 Directory structure
• TypeScript support #18780 TypeScript support

[epixa]

Now that most of this is finished, I'm going to close this meta issue out in favor of tracking the trail of other important UI work as discrete issues.