refactor static and asset emission to be actual emitter plugins

quartz/plugins/emitters/index.ts

@@ -3,3 +3,5 @@ export { TagPage } from "./tagPage"
 export { FolderPage } from "./folderPage"
 export { ContentIndex } from "./contentIndex"
 export { AliasRedirects } from "./aliases"
+export { Assets } from "./assets"
+export { Static } from "./static"