Speed up compiling internal packages for faster branch switching

[nounder]

Jumping across branches with difference dependencies and internal package is currently very inconvenient.

On branch change I need to npm install (which on a 2nd run is fast thanks to npm cache) and then npm build which runs build on all apps and packages. This process is really slow because of daimo-web which builds whole Next.js app. On my Air M1 it takes more than 20s.

---

Decouple daimo-web build process from compilation process from package. We can compile packages automatically on npm install by adding postinstall script. This way, after changing a branch, it will be enough to run npm install to update external deps and internal packages.

[dcposch]

To compile without web:

npm run build:api

[dcposch]

Good issue for updating the README though