The mobx-state-tree examples (with and without typescript) pre-dated the next _app class and needed to be updated to use _app for persisting state across client-side navigation transitions. Also removed unneeded custom server class to better keep with the "show one feature per example" style of the next examples folder
* Always fetch pages with '.js' extention from client side.
* Load error page always from _error.js rather _error/index.js
* Load pages from page.js instead of page/index.js from the client for static exports.
* Update index.js
* Simplify the path re-write logic in the webpack pages-plugin.
* Adapt with-mobx example for with-mobx-state-tree
* Remove unnecessary lastUpdate parameter to show off snapshot
* update readme
* make other.js more closely mimic index.js
