feat: assign filetype to background buffer

[ditsuke]

Summary

Assigns a filetype (zenmode-bg) to the background buffer created by zen-mode.

Why?

I use a utility to close floating windows. This utility interferes with zen-mode's background window so it helps if it has a filetype I can use to filter it out.

[github-actions[bot]]

This PR is stale because it has been open 60 days with no activity.

[folke]

ty!

[ditsuke]

Thanks for merging!