Modify/create index files to handle exporting

src/file/checkbox/index.ts

@@ -0,0 +1,3 @@
+export * from "./checkbox-util";
+export * from "./checkbox-symbol";
+export * from "./checkbox";

src/file/index.ts

@@ -17,3 +17,4 @@ export * from "./track-revision";
 export * from "./shared";
 export * from "./border";
 export * from "./vertical-align";
+export * from "./checkbox";