Capture NVIM_APPNAME on Windows during batch script installation

[Beastwick18]

When creating the .sh script for Linux, "NVIM_APPNAME" is set as expected. This does not happen on Windows, however. I've added a new function, s:capture_windows_env_var, which adds a statement formatted like:

if NOT DEFINED some_variable (
  set "some_variable=some_value"
)

The only environment variable added to the batch script is NVIM_APPDATA

[glacambre]

Awesome, thank you for taking the time to implement this! :)