add options in chokidar to use polling

- fixes sveltejs#109 
- chokidar is a transitive dependency of livereload, so passing an option to chokidar is relatively safe
- the fix was inspired by rollup/rollup#1666 (comment)
- tested on both macOS and Ubuntu 19 (via WSL2)

rollup.config.js

@@ -49,7 +49,10 @@ export default {
 		production && terser()
 	],
 	watch: {
-		clearScreen: false
+		clearScreen: false,
+		chokidar: {
+			usePolling: true
+		}
 	}
 };