Formatting the release history with headings makes it much more scannable (for me, at least), and also helps developers using assistive technology to scan the page. It also makes the notes for individual releases linkable.

In #125, we switched the default aync interface to promises/async/await, but missed updating the module-level tests. This updates them.

This attaches to the process’s `beforeExit` event (supported in Node.js, Deno, and Bun) when auto-flushing is enabled in order to automatically flush when a program exits. Before, you needed to remember to call `metrics.flush()` manually at the end of your program, which is easy to forget (or not even know about in the first place!).

It also adds a `stop()` method that disables auto-flushing and flushes any remaining buffered metrics.

Fixes #136.

Release v0.12.1 with some maintenance updates and the new `process.beforeExit` functionality from #141.