Save reference to watcher in FileSystemLoader for async shutdown

[lcrosetto]

FileSystemLoader does not keep a reference to the watcher:

nunjucks/nunjucks/src/node-loaders.js

Line 43 in ea0d6d5

const watcher = chokidar.watch(paths);

It would help to save the reference (to enable synchronous shutdown of chokidar with watcher.close()), as is done in NodeResolveLoader:

nunjucks/nunjucks/src/node-loaders.js

Line 101 in ea0d6d5

this.watcher = chokidar.watch();