Information on required Visual Studio versions and Python versions
were out of date, considering that we use newer `node-gyp` now.

Python 3 will be supported on Windows when atom/atom#20856
or similar gets merged into the atom repository.

This should be a recommended way of getting build tools set up.
It works really well, and is much easier than
a manual install of Visual Studio/C++ tools/Python 2.

It sets them all up for you. I personally wouldn't
recommend any other way to someone I knew.

Co-authored-by: Amin Yahyaabadi <aminyahyaabadi74@gmail.com>

Visual Studio 2015, 2017 and 2019 are supported.
Windows 8 SDK or Windows 10 SDK are supported.
Clarified windows-build-tools install instructions.
The latest windows-build-tools will be fine for the foreseeable future
(Even when they default to Visual Studio 2019 some time soon.)

Update install instructions for Windows.

This is now a dependency "7zip-bin" in `scripts/package.json`.

The script doesn't look for 7-Zip on the system anymore,
so it wouldn't be used even if installed.

This should be the first option. It is recommended by node-gyp and Microsoft.

It is either officially or unofficially developed and maintained by Microsoft themselves.

- Suggest installing windows-build-tools v4,
as v5 can hang and fail to install.

- Don't use the `--production` flag to install
windows-build-tools; Not installing devDependencies
is already implied when installing a package by name.

Building Atom with Visual Studio 2019 causes a few CI tests to fail,
so hold off on recommending Visual Studio 2019 for now.

This version of npm includes newer node-gyp,
which is better at automatically detecting Python
(especially Python 3), and also better at 
automatically detecting Visual Studio.