fix(types): '_filePath' implicitly has an 'any' type

Author: ulivz

packages/@vuepress/types/lib/context.ts

@@ -15,7 +15,7 @@ export interface Page<
   /**
    * file's absolute path
    */
-  _filePath;
+  _filePath: string;
   /**
    * Access the client global computed mixins at build time, e.g _computed.$localePath.
    */