Fix Windows build

[rmitsch]

Resolves #1.

This fixes the build on Windows by including the SQLite source code and considering it in setup.py. Vendoring SQLite might not be the best option here - downloading from https://sqlite.org/download.html might be more reasonable - but I think it's probably fine for now.

A CI with a smoke test would probably a good idea, I'd include that in a separate PR though.

[karlb]

Sorry for the late reply.

I don't understand how this works and why this is necessary.

• Don't the Windows compilation instructions on https://www.sqlite.org/loadext.html apply for spellfix?
• Do you have any explanation or link on why the normal approach doesn't work and why your approach solves the issue?
• I see that sqlite/windows.c is included when building on Windows, but I don't see how any of the other added files are ever used.