Add events for core telemetry

[krschau]

Summary of the pull request

Add events for measuring:

• % devices with Dev Home crashes
  • adds DevHomeStartedEvent to replace placeholder currently used
• Dev Home regular startup success rate
  • New event: DevHomeStartedEvent
  • New event: DevHomeShellLoadedEvent
  • New event: PageLoadedEvent
  • We'll measure only instances where DevHomeShellLoaded tells us we started with a "Launch" activation kind, and measure if we successfully got to PageLoadedEvent for Dashboard or What's New page.
• Dev Home regular startup boot time
  • Time from DevHomeStartedEvent to PageLoadedEvent. Exclude any time taken by store download, measured between DevHomeInitializationStartedEvent and DevHomeInitializationEndedEvent.

References and relevant issues

https://task.ms/52413149

Detailed description of the pull request / Additional comments

Validation steps performed

PR checklist

• Closes #xxx
• Tests added/passed
• Documentation updated