fix(info): verify moddable sdk git repo, include nrf52 support

[HipsterBrown]

Discovered by @mrpeu, the CLI assumes the directory for process.env.MODDABLE is a Git repository to be able to determine the Moddable SDK version or branch, as well as pull updates for the SDK. This PR adds an extra check to confirm it is a repo before attempting any git commands.

While working in the doctor command module, I noticed support for nrf52 was never added 😬 , so that has also been added in this PR.

xs-dev environment info:
  CLI Version                0.30.2
  OS                         Darwin
  Arch                       arm64
  Shell                      /bin/zsh
  NodeJS Version             v18.16.0 (/Users/nickhehr/.local/share/mise/installs/node/18.16.0/bin/node)
  Python Version             3.11.7 (/Users/nickhehr/.local/share/mise/installs/python/3.11/bin/python)
  Moddable SDK Version       4.3.4 (/Users/nickhehr/.local/share/moddable)
  Supported target devices   mac, pico, nrf52
  Pico SDK Directory         /Users/nickhehr/.local/share/pico/pico-sdk
  NRF52 SDK Directory        /Users/nickhehr/.local/share/nrf52/nRF5_SDK_17.0.2_d674dde