fix: hardcoded '/' in normalized options.fs, fixes #412, #408

Changes including:
- will not call fs.resolve when normalizing `fs`
- removed hardcoded `/`
- mandatory `fs.dirname` for relative reference
- mandatory `fs.sep`, defaults to '/'

src/fs/fs.ts

@@ -1,8 +1,20 @@
+import { LoaderOptions } from './loader'
+
 export interface FS {
+  /** check if a file exists asynchronously */
   exists: (filepath: string) => Promise<boolean>;
-  readFile: (filepath: string) => Promise<string>;
+  /** check if a file exists synchronously */
   existsSync: (filepath: string) => boolean;
+  /** read a file asynchronously */
+  readFile: (filepath: string) => Promise<string>;
+  /** read a file synchronously */
   readFileSync: (filepath: string) => string;
-  resolve: (root: string, file: string, ext: string) => string;
+  /** resolve a file against directory, for given `ext` option */
+  resolve: (dir: string, file: string, ext: string, options?: LoaderOptions) => string;
+  /** defaults to "/", will be used for "within roots" check */
+  sep?: string;
+  /** dirname for a filepath, used when resolving relative path */
+  dirname?: (file: string) => string;
+  /** fallback file for lookup failure */
   fallback?: (file: string) => string | undefined;
 }