move GLFW into sub-repository, use git subtree to sync

[slimsag]

I'm a big fan of monorepos for various reasons, but one huge drawback of them is the fact that one cannot easily:

1. Discover sub-projects within a monorepo
2. Answer the question e.g. "can I use GLFW from Mach? Or is it tied closely to that project for some reason?"
3. Have light-weight dependencies: "I want to use GLFW from Mach, but I don't want to deal with everything else"

Things I like from monorepos include all the standard things:

• It's easy to see development, issues, etc. all in one place
• It's nice to be able to make cross-cutting changes all in one place.
• It's nice to have "one version" for all code, tested together, etc.

This PR moves GLFW into a separate repository https://github.com/hexops/mach-glfw so others can depend on it, discover it, etc. Issues will be in this monorepo, but pull requests can be sent in that sub-repository or here in the monorepo.

git subtree push and git subtree pull are used to synchronize changes: pull requests to this repo are sync'd to that one, and vice-versa. For now this is a manual process I'll do, but can automate later.

Thanks @Avokadoen for pushing me to do this :) #39 (comment)

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[Avokadoen]

Nice!