Can we simplify the windows building by always using `-G Ninja`?

[sbc100]

If we always using ninja we can simplify the windows build and remove all the handling of different MSVC versions?

We could just mandate that if you want to build anything form source you need to run emsdk from visual sudio command prompt. (i.e. you are expected to have run vsvarsall.bat already and/or have cl.exe in PATH).

@juj do you have any reason not to use ninja for all your windows builds?

[sbc100]

Example PR for how much complexity we could remove: #1736