Clean up examples

[jar-stripe]

Why?

In our recent SDK release we added a couple new examples along with a template to make it easy for us to create new examples. The names of the examples and the template were confusing and hard to understand and our documentation around examples was lacking. This PR fixes/clarifies the names and added comments to explain each new example.

What?

• renamed new_example.ts to example_template.ts and added file comment template
• renamed stripe_webhook_handler.js to thinevent_webhook_handler.js and added file comment describing the example
• added comment to meter_event_stream.ts describing the example

[jar-stripe]

Note: this is based off of the jar/fetch-related-object-method branch. Merge https://github.com/stripe/stripe-node/pull/2201 before this PR.