Make snippet watcher more fine-grained

[pokey]

The snippet watcher code is currently triggering a lot of work inside Talon. We'd like to only update the file that changed in the fs.watch

Alternately, we could remove support for custom user snippets dir, but that would mean dropping support for #1306

Originally posted by @lunixbochs in #1348 (comment)

[AndreasArvidsson]

The problem is that snippets can override each other. We have a priority system where typescript reacts wins over typescript and so on. The scenario where if they have removed a definition and we now should fall back to a previous one requires us to keep the content of all files and then figure out what lists have actually updated. Fully doable, but not as simple as just rereading a single file.