Add a simple file watcher to the Cot CLI as a zero-dependency alternative to Bacon

[m4tx]

Context:

Because the template also uses live-reloading on the client (web browser) side, but it's pretty much useless if you don't actually recompile the project. Bacon is currently pretty much a de-facto standard tool for auto recompiling in the Rust world, but maybe we could create our own, simple solution in the future. This way we could avoid adding more configs to the users' projects while it would still be possible to point to Bacon for a more featureful option in the docs.

(#100 (comment))