Implement clang-format

[L1ghtmann]

What does this implement/fix? Explain your changes.

Implements a .clang-format file and does a first pass. Based largely on discussion in #24 and #20. Based on WebKit clang-format.

Checklist

• New code follows the code rules
• I've added modulemaps for any new libraries (e.g. see libactivator/module.modulemap): it should be possible to @import MyLibrary; in ObjC, or import MyLibrary in Swift.
• My contribution is code written by myself from reverse-engineered headers, licensed into the Public Domain as per LICENSE.md; or, code written by myself / taken from an existing project, released under an OSI-approved license, and I've added relevant licensing credit to LICENSE.md

Does this close any currently open issues?

Will eventually resolve #24

Any relevant logs, error output, etc?

Any other comments?

Most edits appear to be a tab->space conversion or destruction of #define formatting. I was unable to find a way to enforce our apparent preference there, but I am quite unfamiliar with header formatting and the various specs/associated tooling.

Changes and opinions are more than welcome! Just wanted to get the ball rolling.

Additionally, we will want to apply an exclusion list (comprised of Apple headers, etc) if/when we implement this into a workflow, as we do for compilation now.

Where has this been tested?

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