Watcher improvements

[lewis6991]

• Use fs_events instead of polling. feat(watcher): move from libuv poll to file event watcher #660
• Share watchers across buffers apart from the same repository. Currently, each buffer gets its own watcher.
• Stop the watchers when nvim is backgrounded. Or better: stop watchers after the first trigger when nvim is backgrouned since any subsequent callbacks aren't doing anything useful.