Add @rancher/shell

[rak-phillip]

This is the first of a series of PR to support Rancher Extensions plugin development for Rancher Desktop.

This PR addresses adding @rancher/shell, resolving compiler issues that arise with the addition @rancher/shell, and introduces an environment variable (RD_ENV_PLUGINS_DEV) that can be used to enable @rancher/shell as the default UI that is loaded for Rancher Desktop. @rancher/shell can be enabled by running $ API={ $rancher-manager-url } RD_ENV_PLUGINS_DEV=1 npm run dev.

Currently, some sort of Rancher Manager backend is required for the API variable, I have been making use of my hackweek project https://github.com/rak-phillip/dorm to quickly provision Rancher Manager in Digital Ocean. This requirement is entirely an interim step and will be removed once we modify @rancher/shell to function without any backend. Removing this requirement will allow us to render a UI while we wait for Kubernetes to start or when Kubernetes is disabled in Rancher Desktop.

closes #3143

[ericpromislow]

Note that with this PR Rancher Desktop won't work when the env vars are both set. This is a step to building a true plugin architecture