This PR fixes an issue where the thread history would only work in one
Zed window at a time.

The backing LMDB database can only be opened once per Zed instance.
However, the `ThreadStore` has one instance per Zed window.

To fix this, we need to create the `heed` environment once and store it
as a global, and then reference the same environment across all of the
`ThreadStore`s.

Release Notes:

- N/A