fix(router): include ctx-shared in package files

[chriszs]

Why

Fixes the failure documented in expo#27740 wherein typed route generation failed to find an import called ctx-shared in the Expo Router canary package.

How

Includes the ctx-shared.js file used by typed route generation in the expo-router package files by adding it to the explicit list of included files in package.json.

Test Plan

To test I started with my minimal reproduction (in which npm start fails), downloaded ctx-shared.js, moved it into node_modules/expo-router/ and then ran npm start again (successfully).