fix: restore browser support

Author: BelfordZ

src/index-web.ts

@@ -4,4 +4,5 @@ import fetch from "isomorphic-fetch";
 export default buildReferenceResolver(fetch, {
   access: () => Promise.resolve(false),
   readFile: () => Promise.resolve(""),
+  constants: { F_OK: 0, R_OK: 0 } //tslint:disable-line
 });