Add generic file watcher api

[pokey]

• Required by Query hot reloading during development #1803
• Required by Scope support tree view #1663

Checklist

• I have added tests
• I have updated the docs and cheatsheet
• I have not broken the cheatsheet

[AndreasArvidsson]

I'm not in love with the polling implementation. I would prefer if we added support for file watchers if you have a recent enough version.

[pokey]

I'm not in love with the polling implementation. I would prefer if we added support for file watchers if you have a recent enough version.

Yeah I think probably not too much effort to do version check and use watcher if available. For now, this is only used for dev hot reloading, though, (ie the code doesn't even run in production), and I won't have much time to work on it for next few days as I'm travelling. Mind if we merge this so you / @josharian can have hot reloading and then I'll do quick follow-up early next week with proper watcher?

Fwiw this is the exact same mechanism we use in production for snippet reloading, and it's quite low resource utilisation as it's just a stat

[josharian]

Mind if we merge this so you / @josharian can have hot reloading and then I'll do quick follow-up early next week with proper watcher?

yes please!

[josharian]

tbh, for dev only, i don't see that it's worth the effort to switch from polling, if polling is already built.

[pokey]

We will be using it for the tree-view next, so I'll switch before we ship that

[pokey]

You can also just rebase onto the hot reloading PR while developing for now. That's what I did for my other PR

[AndreasArvidsson]

How can I say no to josh? Commence the merging!