feat: hot reload, fixes #82

[a-h]

No description provided.

[joerdav]

Worth a look, github have a fork of the browser package imported in this PR that is actively maintained and updated:
https://github.com/cli/browser

[a-h]

I'm getting fairly happy with this now. I've been doing some work to reduce the CPU utilisation, since I noticed it was using a lot of CPU while watching.

I added profiling, and noted that walking the directory every 250ms caused a lot of syscalls, and switched to WalkDir.

However, I think it's still too much. Next, I thought that I'd try out a phased approach:

• On start, walk all directories and compile a list of files.
• Every 250ms, check the existing set of found files for changes, and maybe do a backoff so that it will wait for up to 1000ms (not sure on the right value yet) to check files that haven't changed recently.
• Every 1000ms-2000ms, re-walk all directories and update the list of found files.

[joerdav]

It would be more work to maintain, but you could use a separate technique for systems that support inotify rather than polling.

Or if you're willing to take on another dep:
https://github.com/fsnotify/fsnotify

[a-h]

I tried out using fsnotify and friends, but couldn't get it to work well on my machine.

Here's an abandoned branch - https://github.com/a-h/templ/tree/hot_reload_watcher

I'm not sure I want to support the weird errors I'll get with a filesystem watcher implementation, since it requires me to have a machine for each OS and to test it out.