Consider using os.homedir() instead of process.env.HOME

[cvaldev]

Is your feature request related to a problem? Please describe.
While working on #4449 I noticed that inside VimrcImpl we're currently using process.env.HOME to refer to the current user's home directory. This is not platform agnostic as it returns undefined on windows systems. On windows systems we'd have to use process.env.USERPROFILE.

Using process.env.HOME on windows means we can't support expanding ~ or $HOME, nor find the default vimrc.

Describe the solution you'd like
We should switch to os.homedir() as it resolves the home directory correctly regardless of platform.

Additional context
Although I'm not a windows user I did my own testing to verify the behaviour on windows, plus here are some links that support the above.

types/_generator-typings#91
Node.js Find home directory in platform agnostic way