Make toolkit build dynamically select modules to build

[rikukissa]

Description

Take a look at this PR by @Zangetsu101 . Every time we add a new module, we need to change the build scripts, which is not ideal.

Can we make the build script automatically pick up any modules inside src and build them all?

It might be worth exploring if we could for instance use rollup or some other tool for building this.

Bonus: add a "build:watch" task that is started when yarn dev is run.