Overhaul terminal output with gluegun

[irskep]

Fix #8.

I'm really not crazy about gluegun as a whole, but it's the only thing I can find with a sane API and behavior for showing a spinner and printing some output.

The general idea is that most operations are shown with a spinner, and if there were any interesting logs, print them after the operation is over.

> bun src/cli.ts docs --local
✔ Parsing documents
✔ Setting up plugins
✔ Highlighting code blocks
✔ Transform pass 1 of 1
✔ Connecting pages
✔ Copying static files from templates/html to docs/out/html
✔ Copying static files from docs/src to docs/out/html
✔ Rendering html
    Not sure what to do with link contributing/building_and_running#indexterm-4 in plugins/indexterms