Fix circular import in background.ts
Fix circular import in background.ts

RollupJS reports a circular dependency:

Error: Circular dependency: node_modules/csx/lib.es2015/index.js -> node_modules/csx/lib.es2015/background.js -> node_modules/csx/lib.es2015/index.js

basarat/csx,basarat/csx,basarat/csx,typestyle/csx