Remove finalhandler dependency

[tmcw]

This dependency contributes a bit of bulk to this package: https://npmgraph.js.org/?q=@11ty/eleventy@4.0.0-alpha.7#select=finalhandler%402.1.1

And appears to be unnecessary, and kind of an inconvenience even - it has one bug with a workaround and an unwanted CSP policy. I think it could be removed and replaced with the barebones logic that we want instead.

[ajiho]

https://pkg-size.dev/browser-sync

https://pkg-size.dev/@11ty/eleventy-dev-server@3.0.0-alpha.8

Actually, @11ty/eleventy-dev-server is already much lighter than browser-sync, and judging from the current state of the browser-sync repository, it feels somewhat stagnant. On top of that, browser-sync’s feature set is a bit too heavy for my needs, while @11ty/eleventy-dev-server feels just right and very comfortable to use. Of course, it would be even better if it had even fewer internal dependencies.