Automate installation of plugins

Having to install both Dart and Flutter plugins, and remember to restart, can be a bit rough. We should consider installing the plugins as part of the getting started flow.

This should be possible by unzipping the plugin zips into the [plugins folder](https://intellij-support.jetbrains.com/hc/en-us/articles/206544519-Directories-used-by-the-IDE-to-store-settings-caches-plugins-and-logs).