**Reason**
- Main way to develop under Apple's platforms is using SPM

**Changes**
- Added Package.swift
- Added _SwiftPackageManagerFile.cpp to single_include/nlohmann in order SPM to see it as a C++ source file
- Added .build to .gitignore

**Context**
- SPM is a package manager for Swift and C-based languages. It is the main way to develop under Apple's platforms. It is also used to build Swift packages for Linux.