### Describe the problem that is addressed by this PR
Adds a script `init.sh` to bootstrap new Codespaces. At the moment, the
script does the following:
- Installs Node LTS based on the version in `.nvmrc` and sets it as the
default Node version
- Installs Rush and Autorest
- Runs `rush update`
- Installs Powershell (for running engsys scripts and asset sync
migration)
Timo van Veenendaal