Join the license header with historical comments using a separator so IDEs can correctly parse the initial header.
Also use .gitattributes to ensure all files are LF.
At this point, all remaining references to `MACINTOSH` are either part
of multi-value conditionals that `unifdef` was unable to determine the
final value for, or a `#define`, or a comment. I manually go through the
files with a Ctrl+Shift+F search (ignoring legacy/, to preserve useful
historic knowledge) and clean up each usage.
As part of that search, I discovered `lib/mac/` which I deleted wholly.
Test Plan:
On Mac, build both Debug and Release
```
cmake --build --preset mac --config Debug
cmake --build --preset mac --config Release
```
