Introduce nob build

[mikmart]

This PR introduces a nob build and uses it to add MSVC support.

Motivation:

I wanted to add an MSVC build for the B compiler. It would be convenient with my WIP MSVC codegen plugin so you can work on Windows without any GNU toolchain. Trying to do that in the Makefile was a huge mess.

Since we already require both a C compiler and nob.h to build the B compiler, it seemed like a natural choice to introduce a nob build that could eventually replace the Makefile.

Cons:

• No incremental builds.

Pros:

• No incremental builds.
• No dependency on make -- one less moving part.
• Real programming language to handle build complexities.

Tested on:

• Windows 10 with TARGET_MSVC
• WSL Ubuntu with TARGET_MINGW32
• WSL Ubuntu with TARGET_POSIX

I know you've said on stream and in #283 that you'd prefer to extend bgen for the build. But as bgen still needs to be bootstrapped and the C files need to be compiled, I think switching to nob for that would still have tangible benefits.

What do you think?

[nullnominal]

I think ./nob should bootstrap, then ./build/bgen should do the rest

[mikmart]

That makes sense conceptually. But consider that the bgen bootstrapper needs to be able to:

1. Build the thirdparty objects.
2. Build a Crust executable.
3. Do 1-2 using for the toolchain/OS of your choice.

Those are the same capabilities that are currently required to build b and btest. By necessity the bootstrapper has them, but right now bgen doesn't. So at least right now I don't see the benefit of asking bgen to learn to do those as well.