Add Extension Telemetry

[iampeterbanjo]

As an extension developer I would like some data about the use of this extension that would be useful in guiding future development.

Consider:

• What kind of data would be useful to collection?
• Should this be an opt-in or opt-out?
• How do other extension do this?

Useful references:

• How to perform telemetry for extensions microsoft/vscode#18035
• https://github.com/formulahendry/vscode-extension-leaderboard/blob/master/src/appInsightsClient.ts