feat: support multiple extensions for routes file

[ScriptedAlchemy]

This pull request includes a change to the pluginReactRouter function in the src/index.ts file to improve the way the routes file is located and to generalize error messages.

• Changed the method for locating the routes file from resolve to findEntryFile to allow for more flexibility in finding the entry file.
• Updated error messages to be more general, replacing specific references to routes.ts with routes file.

#15