`make` `build-if-not-exists` and `build-if-changed` always build

That's because they look for `dist/app.js` but webpack uses `fullhash` in the name, i.e. `dist/app.20fd562f4b5b27e29042.js`

https://github.com/Automattic/simplenote-electron/blob/f5d463f7a5f5beb3638696c9700b1c7781448744/Makefile#L21

https://github.com/Automattic/simplenote-electron/blob/f5d463f7a5f5beb3638696c9700b1c7781448744/webpack.config.js#L23

Start with the linked Makefile entry around line 21 and compare its output check with the filename pattern in webpack.config.js around line 23. Run the `build-if-not-exists` and `build-if-changed` targets before and after a build, and confirm they recognize the generated hashed bundle instead of rebuilding every time.

Written by the indexing model from the issue text.