fix: make filePaths platform agnostic

Use `path.sep` as the file separator instead of `/` so that this
works on both Windows and Unix.

Rename input parameter from `path` to `modulePath` so that it
doesn't override `node:path`

src/index.ts

@@ -250,8 +250,8 @@ const isFile = (path?: string | undefined): path is string => {
   }
 }
 
-const isModule = (path?: string | undefined): path is string => {
-  return !!path && isFile(`${path}/package.json`)
+const isModule = (modulePath?: string | undefined): modulePath is string => {
+  return !!modulePath && isFile(`${modulePath}${path.sep}package.json`)
 }
 
 /**